Preheat oven to 350°F.
Reserve 6 asparagus spears. Cut remaining asparagus at an angle into 1-inch pieces, set aside.
Sauté bell pepper in 2 tablespoons olive oil until soft, but not browned, for 7 minutes.
Stir in onion and asparagus pieces; sauté for 1 minute. With a slotted spoon, remove vegetables to drain on paper towels; set aside.
Whisk chopped parsley, salt, and pepper into beaten eggs. Stir in cheese and reserved sautéed vegetables. Coat the inside of a heavy, non-stick, 12-inch sauté pan (with a cover) with softened butter.
Pour egg mixture into pan, cover with lid. Bake at 350°F until eggs are just firm, 35 minutes. Remove cover; bake for 10 minutes more, until top is lightly browned.
Loosen frittata, then cover pan with a large, warmed serving platter. Flip frying pan over onto platter. Cut frittata into 6 wedges; garnish each with 1 reserved asparagus spear and parsley on top; lemon wedge on the side.
Source: California Asparagus Commission