2 tablespoons butter
1/2 cup finely minced yellow onion
1 small apple (peeled and coarsely shredded)
1 tablespoon fresh thyme
1 cup fresh, soft bread crumbs
1/4 cup milk
1 1/4 pounds ground turkey (20 ounces)
2 tablespoons Dijon or honey mustard
1/2 cup grated Parmesan cheese
2 tablespoons fresh minced parsley
1 tablespoon fresh chopped tarragon
1/2 teaspoon salt
1/2 teaspoon freshly ground pepper
  olive oil (for the grill)
  sliced Gruyère cheese (optional)