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.