Heat the oil in a skillet over medium heat, and cook the onion and garlic until tender. Mix in mushrooms, green bell pepper, and tomatoes. Season with rosemary, salt, and pepper. Stir in tomato paste and beef broth. Reduce heat to low, cover, and simmer 15 minutes.
Bring a large pot of lightly salted water to a boil. Add spaghetti and cook for 8 to 10 minutes or until al dente; drain.
Mix the flour into the sauce to thicken. Serve sauce over the cooked pasta.
