How to Make It
In a medium skillet, heat 2 tablespoons of the olive oil. Add the onion and minced garlic and cook over moderate heat until softened, 8 minutes. Stir in the thyme and rosemary and season with salt and pepper. Transfer to a large bowl and let stand until cool, about 10 minutes.
In a small bowl, soak the bread pieces in the milk until absorbed, about 5 minutes. Squeeze the milk from the bread and transfer the bread to the bowl with the onions. Add the veal, egg, Parmigiano-Reggiano and 2 1/2 teaspoons of kosher salt and 1/2 teaspoon of black pepper. Mix with clean hands until well combined.
Scoop 1 1/2 tablespoon-size balls from the veal mixture. Stuff a cube of Fontina into the center of each meatball and roll to enclose the cheese. Transfer the meatballs to a rimmed baking sheet and refrigerate for 30 minutes.
In a large nonstick skillet, heat 2 tablespoons of the olive oil. Cook the meatballs in batches over moderately high heat until browned all over, about 6 minutes. Return the meatballs to the baking sheet.
In the same skillet, heat the remaining 1 tablespoon of olive oil. Add the sliced garlic clove and cook over moderately high heat until it is lightly golden, about 3 minutes. Add the tomato paste and cook, stirring constantly, for 1 minute. Add the diced tomatoes and bring to a simmer. Add the meatballs and any accumulated pan juices to the tomato sauce. Cover and cook over moderately low heat, turning the meatballs once, until they are cooked through, about 10 minutes.
Using a slotted spoon, transfer the meatballs to bowls or plates. Simmer the sauce over moderate heat until thickened slightly, about 2 minutes. Season with salt and pepper and pour over the meatballs.