Fill a large pot with 5 liters of water and cook on high heat.
Once the water starts boiling, add 35 g salt and the Penne Rigate. Stir with a spoon and let the pasta boil for 2 minutes less than the time indicated on the package.
Strain the pasta and drizzle with 2 tbsp. olive oil. Stir gently and leave the pasta aside.
Heat 1 tbsp. olive oil in a frying pan and sauté the onions on medium heat, for 4-5 minutes until they soften. Add salt and pepper and allow to cool.
In a large bowl mix the cream with half the quantity of the cheeses, the nutmeg and freshly ground pepper. Add the pasta, cherry tomatoes and sauteed onions into the bowl and mix gently.
Transfer the contents of the bowl to a lightly oiled ovenproof dish and spread evenly. Sprinkle with the remaining cheeses.
Bake the pasta in a preheated oven at 180°C for approximately 30 minutes, until the cheese has melted and is golden brown.
Allow the soufflé to stand for 7-8 minutes and then serve.