Thinly slice the onion. Cut the bell peppers in half lengthwise, remove the stems and seeds, then slice thinly.

Boil a pot of water, add salt (not listed in ingredients), and cook the spaghetti according to package instructions. Reserve 3 tbsp of pasta water.

Heat a frying pan, add the tuna (including oil) and onion, and sauté over medium-low heat until the onion softens.

Add ketchup to the pan and cook until the moisture evaporates, enhancing the flavor.