Heat half the butter, 2 tablespoons of stock and the shallots in a small pan. Cook covered over a medium heat for 5 minutes or until the shallots have softened, stirring occasionally. Add the lettuce leaves and cook, stirring for 2 minutes.
Add the peas and half the remaining stock to pan, bring to the boil, reduce heat and cook, covered, for 7 minutes or until the peas are tender. Add a little more stock if the sauce dries out. Season to taste.
Cook pasta in a large pan of boiling water into al dente. Drain well through the remaining butter in a serving bowl, spoon over the sauce. Sprinkle with mint and serve with Parmesan.