Be careful not to brown the onions too quickly - they will continue to brown as they cook with the bread. From Cook's Countrymore
# asparagus, tough ends snapped off, spears cut diagonally into 1-inch pieces
T unsalted butter
small onion, minced
slices hearty white sandwich bread, cut into 1-inch squares
cup Boursin cheese, crumbled
Adjust oven rack to middle position and heat oven to 400 degrees. Toss asparagus with 2 T water in microwave-safe bowl. Cover with plastic wrap and microwave on high power until crisp-tender, about 2 minutes. Drain. Whisk eggs, milk, and 1/4 t pepper together in large bowl. Melt butter in large nonstick ovensafe skillet over medium-high heat. When butter foams, swirl to coat skillet and add onion and 1/2 t salt. Cook until onion is soft, about 3 minutes. Add bread and cook, stirring frequently, until bread and onion are lightly browned, about 3 minutes. Off heat, stir in asparagus and egg mixture until well incorporated. Scatter cheese on top and bake until top is puffed and edges have pulled away from sides of pan, 15 to 20 minutes.