To make the meatballs, combine the beef, cheese, bread crumbs, garlic, basil, parsley, pepper flakes, salt and pepper in a large bowl. Mix until just combined (don’t overmix!) and them form meatballs that are a little less than 1-inch in diameter. Heat a large skillet over medium-high heat and add the olive oil. Add the meatballs and cook until they are browned on all sides (don’t worry about them being cooked through yet), about 5 minutes total.
Reduce the heat to medium-low, add the diced tomatoes, butter, another pinch of salt and pepper and stir. Cover and let simmer for 15 minutes. Now, do what you want with your meatballs! Like make grilled cheese.
To make the grilled cheese, spread the outsides of the bread with a 1/2 tablespoon of softened butter. Heat a skillet or griddle over medium heat. Add some grated fontina to the bottom slice of bread, cover it with a few meatballs (I sliced mine in half), add a bit more fontina and another bread slice. Cook the sandwich until the cheese is melted and the bread is golden and toasty. Serve immediately!