In a large pot of boiling salted water, cook pasta until just al dente. Drain well and toss with 1 tablespoon of olive oil.
Preheat oven to 180 C / Gas 4. Grease an ovenproof baking dish.
In the meantime, melt butter in a medium saucepan over medium heat. Add the onions and cook and stir until tender and translucent. Add tuna and cook for another 2 minutes. Pour cream over tuna, and season with salt, pepper and nutmeg.
Place pasta in the saucepan, toss well and and transfer to the prepared baking dish. Sprinkle with Parmesan cheese.
Bake in the oven for 15 to 20 minutes until golden brown on top. Remove from the oven and serve.