Pour off all but 1 tablespoon of the bacon fat from the skillet and add 2 tablespoons of the butter. Add the onion and carrot and cook over moderate heat, stirring, until the carrot is crisp-tender, about 5 minutes. Add the peas and cook just until heated through. Add the chicken stock, cover and simmer until the carrot is tender, about 3 minutes. Stir in the shredded lettuce, lemon juice, parsley and bacon and cook just until the lettuce wilts. Season the vegetables generously with salt and pepper and keep warm.