Melt a good knob of butter in a saucepan, then gently sauté the onion in the melted butter. (Adding a pinch of salt and a drop of water softens the onions as they cook so they don’t burn.)
Add the rice and let it ‘toast’ in the onion for a few minutes, stirring from time to time, until the grains turn chalky white. Take care that neither the onion nor the rice browns.
Add a splash of the wine. Let the wine evaporate as you stir the rice.
Now add the broth, ladle by ladle, allowing each ladleful of broth to evaporate before adding the next. Stir from time to time. Repeat until the rice is al dente, which should take around 18-20 minutes.
When the rice is cooked, take the pan off heat. Add a half ladleful of broth (or water) and the mixed cheeses and another knob of butter. Stir vigorously until the rice reaches as uniform, creamy consistency.
Serve immediately.