Peel and slice the onion into strips. Wilt it gently in a pan with a splash of oil, then add the peas. Season while stirring, season with salt and pepper and add a cup of water. Cover and cook over medium heat for about 10 minutes from boiling.
Remove a few spoonfuls of peas along with the onion and set aside. Instead, add the cream, a grating of nutmeg and butter to the pan with the remaining peas. Cook for 5 to 10 minutes.
Transfer everything to a long, narrow mug, then whisk to make a thick velouté. Keep aside.
Cook the tortellini in boiling salted water, drain and transfer to a bowl, season with a drizzle of oil and add the peas kept aside. Stir.
Spread the cream of peas in two bowls, continue with the tortellini.
Finish with parmesan petals on both plates and serve immediately.