In a large saucepan of salted boiling water, blanch the beans and peas separately until crisp-tender, about 20 minutes for fresh limas, 5 minutes for sugar snaps and fresh English peas and 2 minutes for snow peas and frozen limas; frozen petite peas won't need to be blanched. Rinse well with cold water after cooking, then drain, shaking off the excess water, and transfer to a large bowl.
Melt the butter in a large saucepan. Add the chicken stock and cook over high heat until reduced by half, about 4 minutes. Add the beans, peas and romaine lettuce and cook, stirring, until the lettuce is wilted and the peas are heated through. Season with salt and pepper and serve at once.