Cut the onion into thin slices.
Melt the butter in the olive oil in a pan.
Add the onion, stir, and season with a pinch of salt or an anchovy.
Sauté the onion gently over a low flame and small heat until translucent or for about 5 minutes. If necessary, add a few tablespoons of water.
Pour the tomato sauce, water, olive oil and continue cooking on low heat for about 15 minutes.
At 5 minutes from the end of cooking time, add the drained and crushed tuna with a fork and stir.
Cook spaghetti al dente in boiling salted water. Drain, season, and serve.