Mix salt, pepper and coffee grounds into the ground buffalo and form 4 burger patties with 1/4 inch thickness.
Brush grill with cooking oil and heat to medium.
Cook burgers for 3-5 minutes per side depending on desired doneness, flipping once. (Because of its low fat content, buffalo will become tough if overcooked.)
After burgers are flipped, add onion rounds to the grill. Grill onions 2-3 minutes per side.
Top each half of the buns with 1-2 tsp oyster sauce. Add a tomato slice (optional) and 1/4 of the grilled onions. Top with a burger and serve open-faced.
If you are only serving 2 burgers, save the second bun and toppings separately, in the refrigerator, for up to 48 hours.
*The recommendation to serve the sandwich open-faced is to help keep the meal light for a romantic evening. If you're going to be serving burgers to a crowd, (particularly outdoors), feel free to use full buns for practical purposes!
(The ground buffalo mixture can also be used to make meatballs. Just brown lightly in a sauté pan and serve with pasta or as an appetizer with oyster sauce for dipping.)