Saute the onions in the olive oil until they soften and change color.
Stir in the peas and add enough broth to just cover them.
Bring to a simmer over medium high heat and cook for 5 minutes or until the peas are cooked as you like them.
Season to taste with salt, pepper and lemon juice.
Stir in the lettuce and allow the heat of the broth to wilt the leaves.