Blanch the collard greens by putting them into a pot of boiling water for 30 seconds.
Strain the boiling water off and quickly transfer the greens to a large bowl of ice water. Let cool, then strain and dry the greens and set them aside.
In a large sauté pan on medium-high heat, melt the butter and oil together. Add onions and garlic, and cook until slightly browned, about 4–6 minutes. Add collard greens and black and red pepper, then cook for 5–8 minutes on high heat, stirring constantly.
Remove from heat, add vinegar, if desired, and stir.