In a large saucepan of salted boiling water, cook the greens for 1 minute. Drain, squeeze out the excess water and coarsely chop.
In the same saucepan, melt the butter in the olive oil. Add the garlic, onion and crushed red pepper and cook over moderate heat until softened, about 5 minutes. Add the mustard greens and cook over moderately high heat, stirring to coat with the flavored oil. Add the stock and season with salt. Transfer to a bowl to serve.