Brush the bread with olive oil and grill it on a hot grill pan. Heat a sauté pan and fry the bacon until it is crisp. Add the onions and fry them until they are tender. Gently mix the peas with bacon and onion and set them aside.
Spread olive oil on the lettuce and grill them on a very hot grill pan.
Bring water and peas to a boil in a small saucepan covered with a lid. Cook them for about 1 minute. Let them cool afterwards in ice-cold water. Drain and blend them with sour cream, salt, and pepper to form the purée. Spread the pea purée and the bacon mixture on the bread. Serve with cheese, grilled lettuce and sprinkle with fresh pea sprouts and a pinch of pepper.
Tip: the pea puree also tastes good with fresh mint and a bit horseradish.