Cut tops off asparagus. Cut stalks into 1/2 inch pieces. Put leeks into a bowl of cold water. Stir around to remove sand. Drain. Cook leeks in 2 T butter in 4 quart heavy saucepan over medium low for 5 minutes. Add asparagus and season with salt and pepper. Cook for approximately 8 minutes. Add broth and cover for about 20 minutes. Puree soup in batches and return to pan. Stir in heavy cream. Add more chicken broth if necessary to bring to desired consistency. Whisk in remaining butter and lemon juice.