Asparagus Soup

2 lbs. fresh asparagus, bottoms snapped off
1/2 to 1 cup chopped onion
4 cups chicken broth
2 Tbsp. butter
4 Tbsp. flour
2 tsp. salt
black pepper to taste
2 cups milk
1 cup sour cream
2 tsp. lemon juice
grated cheddar cheese

Boil asparagus and onion in about 1 cup of the chicken broth until very tender. Transfer to a blender and puree until smooth. Meanwhile, in the pan used for boiling the asparagus, melt the butter. Whisk in flour, salt, and pepper until smooth (don't let it brown!). Whisk in the remaining broth until boiling. Whisk in the asparagus puree, milk, sour cream, and lemon juice. Heat through and serve with grated cheddar cheese.