For fra diavolo sauce, preheat oven to 200°C. Toss ingredients together in a roasting pan and roast until tender and starting to caramelise at the edges (40-45 minutes). Transfer a large spoonful of tomatoes to a plate and discard oregano sprig. Pulse remaining mixture in a food processor to your preferred consistency. Season to taste and keep warm.
Meanwhile, cook pasta in a large saucepan of boiling salted water until al dente (7-9 minutes). Drain, reserving a little pasta water, then return pasta to pan with sauce and 1-2 tbsp reserved water to loosen. Add the reserved tomatoes, season to taste, toss to combine and keep warm.
Heat oil in a frying pan over high heat. Add prawns, tomato paste and oregano, and stir occasionally until prawns are just cooked (2-3 minutes). Add to pasta, adjust seasoning and serve scattered with a pinch of chilli flakes.