Because I don't like throwing away food, I always keep a baggy of the asparagus ends one would normally discard in the freezer. For this soup, you will need more than you think because a lot of it is discarded after it's run through a fine sieve (it's mostly the woody parts, but they are still quite tasty).
Ingredients:

2 cups asparagus stems (I generally use the ends which I have snapped off over the summer months and frozen)
2 cups low sodium chicken stock (or vegetable stock)
½ sweet onion, chopped
2 tablespoons olive oil
salt and pepper to taste
2 slices prosciutto
2 thinly sliced crusty bread
¼ cup grated gruyère

Procedure:

Step 1 Thinly slice prosciutto and fry until crispy in a little oil (1 tsp).

Step 2 Drain crispy prosciutto and set aside.

Step 3 Using about 1-3 tsp olive oil, rub both sides of bread and toast until crispy. Set aside.

Step 4 Using 1 tbsp olive oil, sweat out the onions until soft, add the asparagus stems which have been cut into about 1 inch lengths.

Step 5 Add 1 cup chicken stock and cook until the asparagus is very soft.

Step 6 Using an immersion blender, blend until smooth (you will need a good motor because the woody stems may cause problems).

Step 7 Once you have puréed the soup, ladle into a fine sieve and press through using the back of your ladle - do not skip this step, the soup is not edible because of the woody bits. Add more stock until desired consistency has been achieved.

Step 8 While keeping soup warm, sprinkle the gruyère evenly onto one side of each toast. Place until broiler for a minute until cheese is bubbling.

Step 9 Ladle soup into two bowls, gently top with the gruyère toast and sprinkle with the prosciuto crisps. Serve hot.