1 pound of lean ground beef (feel free to use fattier beef if you prefer)
1 1/2 tablespoon olive oil
1 teaspoon salt
1 teaspoon pepper
2 tablespoons of unsalted butter
12 ounces of sliced baby bella mushrooms
4 ounces fontina cheese, sliced
4 ounces sharp cheddar cheese, sliced
2 cups of fresh arugula
1/2 lemon, juiced
4 whole wheat buns, toasted

mustard aioli
1 1/2 tablespoons dijon mustard
1 large egg yolk
2 teaspoons fresh lemon juice
1/2 cup olive oil
2 tablespoons whole grain mustard
1 tablespoon horseradish
a pinch of salt + pepper