If you love asparagus, this soup is just delicious and, when served with a mixed green salad that’s lightly dressed with a perky-tasting vinaigrette, it makes a fabulous lunch or a casual supper. I suggest using homemade stock, which is basically salt-free and, irrespective of the type, always has so much more flavor than the canned version. That way, you’ll also end up needing less help from things like salt, to get the most intensely satisfying taste. Most important, if using a canned broth, you should only add more salt after tasting the fully assembled soup base. And make sure to puree the solids until very smooth which makes the texture just luscious.
Any time I’ve suggested a tool, a piece of equipment, or a culinary term that’s unfamiliar to you, you can go to Learn to Cook for more information.
Preheat the oven to 375oF. Place the bread slices on a wire rack, over a shallow baking sheet and bake the slices until light golden on both sides, about 20 minutes, turning once. Remove the sheet from the oven and brush each slice evenly, on both sides, with the oil, then sprinkle the tops only, with a little parmesan cheese and some black pepper to taste. Place back into the oven and bake the slices for 5 minutes, then turn the oven off and let the croutons sit there until perfectly crisp, 5 to 8 minutes more. Let the croutons become completely cool, on their rack, before storing them in an airtight tin until needed.
Tie each bunch, in two places, with kitchen twine. Bring a large pot of water to a rapid boil, and add some salt. Submerge the asparagus bundles and boil them until crisp-tender, about 4 minutes. Meanwhile, put a large bowl of ice water on your counter and, when the asparagus are ready, transfer the bundles to the ice water. When cold, remove the asparagus from the water, and pat them dry. Cut off their tips and reserve them separately. Cut the remaining stalks into 2-inch lengths and set them aside for now.
Melt 4 tablespoons of the butter in a 4-quart heavy-bottomed saucepan, over medium heat. Tear off a sheet of wax paper and brush one side with some of the melted butter. Stir the leeks and garlic into the saucepan, coating them evenly with butter. Lay the greased side of the paper directly on top of the vegetables, lower the heat and let them sweat gently, until wilted, about 10 minutes. Discard the paper and stir in the sliced asparagus stalks, diced potato, stock, and dried thyme. Bring the liquid to a full boil, over medium heat, then cover and reduce the heat to low. Simmer the vegetables until tender and the stock is nicely flavored by the asparagus, about 20 minutes.
Place a large wire sieve over a large bowl. Strain the soup through the sieve and reserve the solids. Puree the solids, in batches with some of the liquid, in a food processor fitted with the steel blade, or in a heavy-duty blender, until very smooth. (If using a regular blender, it’s not safe to fill the container more than half full with a hot substance, so, if necessary, do this in several small batches.) As each batch is processed, empty the puree into another bowl and continue until you’ve finished. You’ll end up with one bowl of puree and another, of stock. After rinsing and drying the original saucepan, pour the pureed vegetables back into the pot and add the crème fraiche or cream, if using, along with enough stock to create the desired thickness. (I usually use all of it.) Freeze any remaining stock for another use, but label it “stock, flavored with asparagus.” Season the soup well with salt and black pepper to taste.
(Alternatively, if using a hand-held stick blender, remove a cup or so of the liquid and, insert the stick blender directly into the pot of cooked solids and surrounding liquid. Puree the soup until totally smooth and, when done, add the reserved stock to correct the consistency, if needed.)
Place the roasted peppers in either a blender or a small food processor, fitted with the steel blade. Add the oil, red pepper flakes (if using), and garlic. Puree the mixture until smooth, then add salt and pepper to taste. Transfer the coulis to a bowl or funnel it into a plastic squeeze bottle, snipping the tip so the opening is 1/4-inch wide.
Reheat the soup slowly, over low heat, until piping hot throughout. When almost ready to serve, melt 2 tablespoons butter in a 10-inch heavy-bottomed skillet, over medium heat and, when hot and bubbling, add the reserved asparagus tips. Cook the tips, stirring occasionally, until hot and the edges are beginning to caramelize, about 8 minutes. Place one or two croutons in the bottom of either warmed wide soup plates or deep crocks and ladle the soup on top. Add a generous spoonful of asparagus tips, then spoon in or decoratively squiggle on some of the red pepper coulis. Scatter just a few minced chives on top and serve the soup right away.
The croutons can be made two days ahead and, once completely cool, stored at room temperature in an air-tight tin. Leftovers are fine for several days.
The soup base can be fully assembled two days ahead and kept refrigerated, securely covered.
The soup can also be frozen for several months, when stored in heavy-duty labeled freezer containers. Under these circumstances, however, add the blanched tips directly to the assembled soup base, at the end, without sautéing them.
The coulis and the croutons can be made up to two days ahead. Keep the coulis chilled, but bring it to room temperature before serving. Keep the croutons at room temperature, in an airtight tin.