Bring large pot of water to boil. Coarsely chop the tomatoes and onion. In large skillet, warm the butter and oil over medium-high heat until the butter melts. Add the onion and garlic and saute until the onion just begins to brown, about 3 minutes. Stir in the flour, and cook, stirring until the flour is no longer visible, about 30 seconds. Stir in the tomatoes, clam juice, salt and pepper. Bring the liquid to a boil. Reduce the heat to medium-low, cover and simmer for 10 minutes. Meanwhile, add the pasta to the boiling water and cook until al dente, 10 - 12 minutes, or according to package directions. While the pasta and tomato sauce are cooking, stem and roughly chop the fresh spinich. Bring the tomato sauce to a boil over medium-high heat. Stir in the clams and spinich and cook until the fresh spinich just wilts about 1 minute. Drain the pasta and toss it with the sauce and 1/3 cup of the parmesan. Pass remaining parmesan on the side.