In a large pot over medium heat, heat oil and butter. Saute the onions
until slightly softened, about 2 minutes, then add the red pepper flakes and garlic, cook another minute. Add collard greens and cook another minute. Add the vegetable stock
, cover and bring to a simmer
. Cook until greens are tender, about 40 minutes. Add tomatoes and season with salt and freshly ground black pepper.