Heat the olive oil in a wide sauté pan or shallow pot over medium heat. Add the onion and cook for 6–8 minutes until soft and lightly translucent, not browned. Add the ground beef and cook, breaking it up well, until no longer pink and the moisture has mostly cooked off, about 5–6 minutes. Add the garlic and cook for 30 seconds. Stir in the tomato paste and cook for 1–2 minutes until slightly darkened. Add the passata, salt, black pepper and chili flakes if using. Simmer for 8–10 minutes until lightly thickened. The sauce should taste concentrated and savory, not watery, because it has to season the entire rice mixture. That concentrated base carries seasoned flavour all the way through the rice cooked in it — a watery base would leave the whole mixture correspondingly flat.

Add the rice directly to the tomato-meat sauce and stir well to coat. Add enough hot stock to cover well. Simmer over medium to medium-low heat, stirring frequently and adding more hot stock as needed, until the rice is fully cooked, thick, sticky and much drier than a normal risotto, about 18–22 minutes. You want the grains tender with a little bite left, but the mixture should hold together in a mound and not spread loosely. Stir in the butter, Pecorino Romano and Parmigiano-Reggiano. Taste and adjust salt. This is not a loose risotto. If the mixture is still saucy or flowing, cook it a few minutes longer to drive off excess moisture. That deliberately drier target, well beyond a standard risotto's own consistency, is exactly what gives this rice the real structural firmness needed to shape and hold a filling later.

Spread the rice mixture onto a tray or large shallow dish in an even layer. Let it cool to room temperature, then refrigerate for at least 2 hours, until completely cold and firm. Shaping warm rice will be messy and the supplì will split during frying. The mixture must be fully cold. A thin, flat layer chills faster and more evenly than a mound, which can stay warm in the middle long after the top feels cold.

While the rice chills, cut the mozzarella into 12 pieces. Pat the mozzarella very dry with paper towels. Set up 3 shallow bowls: flour; eggs beaten with 2 tbsp water and a pinch of salt; and breadcrumbs. Damp mozzarella creates steam and can rupture the crust during frying. Dry it well. That thorough drying removes surface moisture that would otherwise convert to steam inside the sealed rice shell, building pressure that risks cracking the supplì open in the hot oil.

Divide the chilled rice mixture into 12 equal portions, about 85–95g each. With damp hands, flatten one portion in your palm. Place one piece of mozzarella in the center. Enclose the cheese completely and shape into a compact oblong or slightly torpedo-like croquette, pressing firmly so there are no cracks. Repeat with the remaining rice and mozzarella. Place the shaped supplì on a tray. Each croquette should feel tight, smooth and compact. Loose shaping causes blowouts in the fryer.

Coat each croquette in flour, shaking off the excess. Dip in the egg mixture, letting the excess drip off. Coat thoroughly in breadcrumbs, pressing lightly so the crumbs adhere evenly. For a sturdier crust, especially if the rice mixture feels soft, dip once more in egg and breadcrumbs. Place the breaded croquettes on a tray and chill for 20–30 minutes. The short post-breading chill helps the coating set and reduces crust separation during frying. That brief chill lets the egg wash begin properly binding the breadcrumb layer to the rice shell beneath it, before the whole croquette faces the sudden shock of hot oil.

Heat the oil to 170–175°C / 340–350°F. Fry the supplì in small batches for 4–5 minutes, turning as needed, until evenly deep golden brown and crisp. Transfer to a rack or paper towels. Let rest for 2–3 minutes before serving. The crust should be evenly golden and crisp, and the center fully hot so the mozzarella melts and stretches. Keep the oil in that range: cooler oil soaks into the supplì before the crust crisps, and hotter oil colours the crust before the centre heats through.

Serve hot. For the classic "al telefono" effect, split one open while hot and pull the halves apart so the mozzarella stretches in long strands. That signature stretch depends on the cheese still being hot and molten — the longer the supplì sits, the more that stretch shortens and eventually disappears altogether.