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1
Preheat the oven to 400 degrees F (200 degrees C).
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2
Heat 1 tablespoon olive oil in a saucepan over medium heat. Add onion and 2 cloves garlic; cook and stir until onion is soft, about 5 minutes.
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3
Place beef in a large bowl. Add onion-garlic mixture, parsley, egg, bread crumbs, and 1 large handful of fontina cheese, reserving the rest for topping. Season with salt, pepper, and cayenne. Mix by hand until just combined. Scoop meat mixture into 24 mini muffin cups; meatballs need not be perfectly round.
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4
Bake in the preheated oven until no longer pink in the centers, about 12 minutes. Leave oven on. Let meatballs cool briefly while you make the garlic butter.
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5
Melt butter in a saucepan over medium heat. Add remaining minced garlic once butter starts to bubble; stir and immediately turn remove from heat. Stir well and set aside.
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6
Line a rimmed baking sheet with foil; coat foil with remaining olive oil. Transfer rolls, still attached, to the baking sheet. Cut a hole in the top of each roll, about 2/3 of the way down. Brush garlic butter over the insides and outsides of the rolls.
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7
Bake until golden brown, about 8 minutes. Pour about 1 tablespoon tomato sauce into each hole. Stuff meatballs into the holes. Top each roll with a spoonful of tomato sauce and remaining fontina cheese. Sprinkle Parmesan cheese on top.
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8
Continue baking until cheese is melted, 12 to 15 minutes. Garnish with more parsley. Let rolls sit for 5 to 10 minutes to firm up; transfer to a serving platter.
