Melt the butter in a saucepan, sweat the chopped onion in it, but do not let it brown. Add rice and let it tarnish. Then deglaze with white wine / prosecco and reduce the heat a little.
Slowly add the chicken soup, stirring constantly (preferably a ladle at a time). However, the rice must never swim.
After 15-20 minutes, pretty much all of the soup should be stirred in. Now add the roughly chopped rocket. After another 5 minutes stir in the butter piece by piece. Then add the prosciutto cut into fine strips. Stir in the parmesan. Season to taste with salt and pepper.