Heat the olive oil in a wide pot over medium-low heat. Add the chopped onions and cook for about 15 minutes until soft, caramelized, and golden. Toward the end, add the sugar.
Stir in the garlic, paprika, oregano, salt (1 teaspoon), pepper, and tomato paste. Sauté for about a minute until fragrant.
Add the crushed tomatoes and the drained tuna, gently breaking it into chunks with your spoon.
Let the sauce simmer on a low heat for 30–40 minutes until it thickens to your liking. If it reduces too much, add a splash of the reserved pasta cooking water (as needed).
While the sauce simmers, cook the pasta according to the package instructions. Before draining, reserve 1- 1½ cups of the starchy cooking water.
Taste the sauce and adjust the seasoning with additional salt and pepper as needed. Toss the freshly cooked pasta with the sauce.