In a bowl combine bison, onion, salt, pepper and garlic powder. Mix well, if mixture appears too dry, add some cold water. Form slider patties using a measuring tablespoon, about 4 Tablespoons for each patty. Place on parchment paper.
In a small skillet, sauté onion in 1 Tablespoon olive oil, until soft.
Preheat a gas grill to medium-high heat. Grill burgers for 1-2 minutes per side, depending on desired doneness.
Add cheese on top of each mini burger right after you remove them from the grill so it can melt.
Lightly toast slider buns and assemble the burgers, adding lettuce, sautéed onion, HEINZ mustard and any additional toppings of your choice.