1 medium Vidalia onion – thinly sliced
Olive oil for sautéing the mushrooms and the onions
1 pound ground beef (85% lean)
Kosher salt and freshly ground pepper
4 hamburger buns (I like poppy seeds on my buns for this burger)
4 slices Fontina cheese
Whole grain mustard
Halve the cremini mushrooms and season with salt and pepper. Add a bit of olive oil to a medium size skillet and saute them until they are lightly browned and cooked through. Add a bit more olive oil to the pan and repeat the process with the onions. Reserve.
Preheat grill for direct-heat cooking. Gently form beef into 4 patties, each 4-1/2 inches in diameter and ½ inch thick (do no overwork meat or patties will be tough). Season generously with salt and pepper. With your thumb, press an indentation in the center of each patty so that they will cook evenly on the grill.
Place patties on grill and grill uncovered, until browned on the bottom, 3 to 4 minutes. Flip, top each with a slice of cheese, cover grill, and cook until cheese is melted and patties are medium-rare, 2 to 3 minutes more. Transfer to a plate. Place buns on grill cut-side down; cook just until lightly toasted and golden brown in places, about 1 minute.
Top each bottom bun with a burger patty. Then top each patty with the sautéed mushrooms, sautéed onions and arugula. Dress cut side of top buns with whole grain mustard and serve immediately.