Heat the olive oil in a pot. Add the onion and the crushed red pepper and cook over moderate heat, stirring, until translucent, 5 minutes. Add the greens in batches, stirring each batch to wilt before adding more. Add the stock and bring to a simmer. Cover and cook over moderately low heat until the greens are tender, 15 minutes. Drain the greens well and return them to the pot.