Peel the lower third of the asparagus and cut off the woody ends. Cut the sticks into short pieces and sauté them with the onions and garlic in hot olive oil. Put in a bowl and mix with the diced tomatoes. Season everything vigorously with salt, pepper and a splash of lemon juice.
Brush the bread slices with a little butter or olive oil and roast them until golden brown on both sides under the grill. Then cut in half and cover generously with the asparagus and tomato mixture. Serve as quickly as possible