Rinse bell peppers, cut in half, remove seeds and ribs and cut into strips. Rinse broccoli, divide into florets, and blanch for 4 minutes in a pot of boiling salted water. Rinse beans, trim, remove ends and blanch for 8 minutes in a pot of boiling salted water. Drain broccoli and beans, rinse with ice cold water and drain well.
Heat oil and butter in a large skillet. Beat eggs in a bowl and season with salt and pepper. Pour eggs into the pan. Distribute vegetables on eggs, cook over low heat and season with salt and pepper. When egg is cooked through, slide frittata onto a plate, cover with cress and serve.