- Portion size 4 servings
In skillet, heat oil over medium heat; fry diced onion, stirring occasionally, until softened, about 5 minutes. Let cool.
In large bowl, beat egg. Stir in thyme, mustard, horseradish, salt and pepper; mix in onion, beef and buffalo. Shape into four 1/2-inch (1 cm) thick patties. (Make-ahead: Cover and refrigerate for up to 6 hours. Or layer between waxed paper in airtight container and freeze for up to 1 month; thaw in refrigerator.)
Place patties on greased grill over medium-high heat; close lid and cook, turning once, until no longer pink inside, about 10 minutes.
Cut rolls in half; place, cut sides down, on grill and toast until golden, 1 to 2 minutes. Layer burgers, lettuce, tomato then onion on bottoms; sandwich with tops.