In a large pot, heat the oil and butter over medium. Add the onion and cook, stirring occasionally, until slightly softened, about 2 minutes. Add the crushed red pepper and garlic, and cook, stirring often, until aromatic, about 1 minute. Add the collard greens and cook, stirring often, about 1 minute. Add the vegetable stock, cover and bring to a simmer. Reduce the heat to low.
Cover and cook, stirring occasionally, until the stems are tender, about 1 1/2 hours. Stir in the tomatoes and season with salt and pepper.