Mash rice/risotto to a soft paste (warm briefly if cold) so it binds.
Sauté onion and bell pepper in 1 tbsp of the oil over medium heat until soft; season with a pinch of salt and red pepper flakes. Cool slightly.
Combine mashed rice, sautéed vegetables, 1/4 cup (15–20 g) bread crumbs, 2 tbsp flour, and 1 tbsp corn starch; mix until cohesive. Adjust seasoning (salt/pepper).
Divide and form 5 balls (or smaller), inserting a mozzarella piece in the center; seal well.
Battering: In a bowl whisk 3 tbsp flour, 2 tbsp corn starch, salt, pepper with enough water (≈60 ml) to form a thick, clinging batter. Put remaining bread crumbs in a separate shallow dish.
Coat each ball in batter (wet hand) then roll in bread crumbs (dry hand). Arrange on a tray. Freeze 10–15 minutes to firm (optional) or freeze longer for later.
Air Fry: Preheat to 180°C / 360°F for 3 minutes. Lightly mist or brush balls with remaining oil. Cook 6 minutes, shake/turn, then another 5–6 minutes until deep golden and crisp.
Deep Fry: Heat 5 cm (2 inches) oil to 175–180°C (350–355°F). Fry 2–3 balls at a time 3–4 minutes, turning, until uniformly golden and cheese just melting. Drain on paper or a rack; season lightly with salt while hot.