Slice the onion thinly. Cut the eggplant in half lengthwise, then slice diagonally into 1cm pieces.

Boil water in a pot, add salt (not listed in ingredients) and cook spaghetti 1 minute less than package instructions. Reserve 50ml of pasta water.

Heat olive oil in a frying pan over medium heat. Add onion and garlic, sauté until fragrant. Add eggplant and cook until coated in oil.

Add tuna, diced tomatoes, reserved pasta water, and seasonings. Bring to a boil, cover, and simmer over medium-low heat for 3 minutes.