Step 1 Heat the olive oil in a large saucepan, then add the onion and cook until translucent, about 1 to 2 minutes. Add the rice and stir until it is well coated with oil. Add the wine and stir until it evaporates.
Step 2 Slowly add the stock a little at a time, waiting for the liquid to incorporate before adding more. Stir frequently. Cook about 20 minutes. The rice should be fairly well cooked, not al dente, but not mush either. Remove from heat and stir in the mascarpone, butter and Parmesan until melted. Allow to cool.
Step 3Spread the risotto into a 9-by-13-inch dish and refrigerate 1 hour until chilled.
Step 4Shape the chilled risotto into 8 patties by hand. Put the flour, egg and panko into 3 separate shallow dishes. Dip each risotto cake into flour, shake off the excess, then dip into the egg, then the panko. Chill 30 minutes.
Step 5Melt the butter in a skillet. Add the risotto cakes and cook on both sides until light golden brown, about 4 to 5 minutes each side. Keep warm until serving time.