In a large pot, heat the oil and butter over medium. Add the onion and cook, stirring often, until slightly softened, about 2 minutes. Add the crushed red pepper and garlic; stir until aromatic, about 1 minute. Add the greens; cook, stirring often, about 1 minute. Add the vegetable stock; cover and bring to a simmer. Reduce the heat to medium-low.
Cook, stirring occasionally, until the collard stems are tender, 40 to 50 minutes. Stir in the tomatoes and season with salt and pepper.