Heat the olive oil in a casserole dish over a low heat, and fry the onion with a pinch of salt for 10-12 mins, or until softened. Add the garlic and fennel seeds, and cook for 1 min more. Stir in the rice and cook for a further minute.
Pour in the tomatoes and half the stock, and turn the heat to medium-high. Cook, stirring continuously, until the stock is almost completely evaporated. Add the remaining stock, a ladle at a time, adding more when the previous addition is absorbed by the rice. Cook until the rice is al dente, about 15-20 mins.
Stir in the parmesan and nduja, then spread the risotto out over a baking tray and leave to cool to room temperature.
Scoop the cooled risotto into 20 even portions, slightly larger than a golf ball. Flatten one of the balls into a disc in your hand, put a piece of mozzarella in the centre, then enclose with the rice. Roll into a ball. Repeat with the remaining risotto and mozzarella.
Tip the flour, eggs and panko breadcrumbs into three separate shallow bowls. Roll each risotto ball in the flour, then the egg, and finally the breadcrumbs. Transfer the balls to a baking tray. Half-fill a large, heavy-based saucepan with vegetable oil and heat over a medium-low heat to 170C, or until a piece of bread turns golden in the oil within 45 seconds. Carefully lower the risotto balls into the oil in batches, and fry each batch for 8-10 mins, or until golden brown and gooey in the centre. Drain on a plate lined with kitchen paper.