Buffalo (Or Beef) Burger - cooking recipe

Ingredients

1 tablespoon vegetable oil
1 onion, diced
1 egg
2 2 teaspoons rosemary or 2 teaspoons oregano (or 1/2 tsp/2 mL dried)
2 teaspoons Dijon mustard
2 teaspoons prepared horseradish
1/2 teaspoon salt
1/2 teaspoon pepper
8 ounces medium ground beef
8 ounces ground buffalo meat
4 kaiser rolls or 4 hamburger buns
4 leaves lettuce
1 tomatoes, sliced
1/2 small red onion, thinly sliced

Preparation

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.