Cook pasta according to package. Drain the pasta and transfer the pasta back into the pot.
Cut onion into slices and drain canned tuna.
Heat cooking oil in a pan over medium heat.
Sauté onion slices until translucent. Add in tuna and capers. Cook the mixture for several minutes.
Add in all-purpose flour and mix it thoroughly.
Pour in heavy cream and stir the sauce until it thickens.
Season the tuna sauce with salt, ground black pepper and ground nutmeg.
Transfer the sauce into the pot of pasta and mix the pasta.
Serve immediately.