Warm 1/4 cup of the olive oil in a small saucepan set over medium-high heat. Add the onions and cook, stirring, until softened, about 3 minutes. Add the tomatoes, crushing them well with the back of a slotted spoon or by squeezing them through your fingers while adding them to the pan. Season with salt and pepper, and stir in the basil. Cover and bring to a boil. Reduce the heat to medium-low and simmer until the sauce thickens, about 20 minutes. Drain half of the olive oil from the canned tuna. Flake the tuna into the tomato sauce. Cover and simmer to heat through, 8 to 10 minutes. Remove from the heat and set aside while the pasta cooks. Bring a large pot of salted water to a boil. Add the pasta and cook until al dente, following the package instructions. Drain, then toss the pasta with the remaining 2 tablespoons olive oil. Add about 3 ladlesful of sauce and continue tossing. Distribute evenly among four dinner plates. Ladle the remaining sauce on top, and serve immediately.