Green peas in ambush

From 1917: Remove the wilted leaves from a large head of lettuce, carefully remove the heart (retain for salad) without cutting through the stalk. Fill with 2 ½ cups of small green peas, add 1 small onion and a sprig of parsley. Sprinkle with salt, pepper, and a half-teaspoon of sugar.
Tie the leaves together with tape and cook 30 minutes in chicken broth or boiling water. Drain and dress with 1 tablespoon of butter and 4 tablespoons of rich cream. Add more salt and pepper. Serve without the lettuce.