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1
Make the meatball mixture: Add the bread to a medium bowl and pour the milk over top. Soak for 15 minutes, then tip off any excess milk. Add soaked bread to a food processor along with the parsley and a pinch of salt, and pulse until a smooth paste forms. Set aside.
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2
Add 1 1/2 teaspoons salt, Calabrian chili, fennel seeds, and 1/4 teaspoon black pepper to a spice grinder and grind until a fine powder forms.
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3
Line the bottom of a large baking dish with parchment paper. Using a large mixing bow, add the ground beef, beef fat, Pecorino, Grana Padano, soaked bread mixture, ground spices, and bread crumbs. Gently mix with your hands until fully combined. Roll meat mixture into 1-inch balls and arrange in a single layer in the prepared baking dish. Cover and refrigerate for at least 5 hours, or up to overnight.
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4
Make the sauce: Heat the olive oil in a large saucepan set over medium-low heat. Add the onion and cook, stirring occasionally, until softened and lightly browned, about 6 to 8 minutes. Add the garlic, fresh chili, and dried Calabrian chili; cook, stirring often, until fragrant, about 2 to 3 minutes.
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5
Add the tomato passata and salt; bring to a boil, then lower the heat to low, cover the pan, and simmer, stirring occasionally, until thickened, about 2 hours.
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6
Cook the meatballs: Preheat the oven to 250u00b0 F (take the baking dish out of the refrigerator and let rest at room temperature while the oven preheats). Transfer the baking dish to the oven and bake until meatballs are firm enough to handle, but still rare on the inside, about 20 minutes. Gently transfer the meatballs to the sauce. Cover and simmer, gently stirring once or twice, until cooked through, about 20 minutes. Taste, and add more salt, if needed. Serve hot.
