1. Mash avocado, sour cream, lime juice and zest, jalapeño, cilantro, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a medium bowl until chunky. Press plastic wrap onto surface of guacamole, and refrigerate.
2. Combine bison, garlic, and remaining salt and pepper in a small bowl. Form into eight 2-ounce patties, transfer to a plate, and set aside.
3. Preheat a grill or grill pan over medium-high heat. Grill sliders until thoroughly cooked; 4 minutes per side for medium.
4. Layer one lettuce leaf, a slice of tomato, and an onion ring on bottom half of each bun, and top with 2 tablespoons guacamole. Add a slider, and top with other half of bun.