Vegetarian "Southern-style" Collard Greens

Slightly modified from Sunny Anderson.
Ingredients:
1 Tbsp. olive oil
1 Tbsp. butter
1/2 large onion, chopped
1 tsp. red pepper flakes
1 clove garlic, finely chopped
1 lb. collard greens, chopped
2 c. vegetable stock
14 oz. can diced tomatoes
Salt
Freshly ground black pepper
Splash of apple cider vinegar

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. Add a splash of cider vinegar before serving.