Melt 25g butter in a large pan and fry onion and garlic until softened.
Stir in the risotto rice and cook until the grains start to turn translucent.
Add wine and simmer until nearly all liquid has evaporated.
Gradually add chicken stock ladle by ladle, stirring until absorbed before adding more.
Add chopped courgettes during the final 3 minutes of cooking.
Stir in mascarpone, 25g parmesan, and 25g butter, then season and chill.
Heat a frying pan with olive oil and the remaining 25g butter.
Pack chilled risotto into a 10cm metal ring in the pan and fry until golden on both sides.
Sauté tomatoes and garlic in olive oil until the tomatoes begin to break down.
Stir basil into the ragu and season with salt and pepper.
Heat vegetable oil in a deep pan to 190°C.
Briefly whisk flour, cornflour, salt, and sparkling water to make a lumpy batter.
Dip sliced courgettes into batter and deep fry until golden and crisp.
Serve the risotto cake on top of the ragu, garnished with parmesan and tempura.