Halve the cauliflower, cut each half vertically into 4 thick pieces and place in a frying pan. Add the stock and bring to a gentle simmer. Cook for 8-10 minutes until tender, turning once. Drain, reserving the stock.
Heat 1 tbsp of the oil in the frying pan and fry the onion for 5 minutes until lightly browned. Stir in the garlic, then the grilled vegetable mix, tomato purée and reserved stock. Bring to a gentle simmer and cook for 6-8 minutes, stirring frequently until thick and pulpy.
Preheat the grill to medium. Combine the remaining olive oil with the chilli, breadcrumbs and cheese. Transfer the vegetable sauce to a shallow heatproof dish and overlap the cauliflower pieces on top. Scatter with the breadcrumb mixture. Grill for 6-7 minutes until the cheese is melting and the breadcrumbs golden.