To Form Patties:

1. In a large bowl, gently break up the ground chuck.
2. Fold in yellow onions, garlic, rosemary, basil, and cayenne pepper.
3. Add kosher salt and black pepper. Mix well.
4. Divide beef into 4 equal patties. Roll each beef portion from hand to hand to form ball, then lightly flatten with your fingertips to form ¾-inch-thick patties.
5. Gently press the center of each patty down with your fingertips until it is about 1/2 inch thick, creating slight depression. Set aside on a platter.

To Grill and Assemble:

1. Using a gas grill, turn burner to high heat and preheat for 15 minutes.
2. Clean grill grate with a lightly oiled rag.
3. Grill brioche buns for 15-20 seconds, then set aside on a plate.
4. Gently place patties on grill. Don’t press them down.
5. Grill for 4 minutes, until well browned on first side.
6. Using a steel spatula, gently flip patties and continue to grill for another 3 minutes for rare, 3 1/2 minutes for medium-rare, and 4 minutes for medium. 
7. Transfer burgers to a plate. Rest for 5-7 minutes before assembling.
8. Layer lettuce, burger, and sliced tomato between each brioche bun. 
9. Serve immediately.