Melt the butter in a large stockpot over medium heat. Cook the onion, bell pepper, and garlic in the melted butter until they begin to soften, about 3 minutes. Add the tomatoes, wine, Italian seasoning, salt, pepper, and garlic salt; cook and stir until the mixture just begins to boil. Reduce the heat to medium low; add the chicken and cook until the meat is tender enough to fall off the bone, about 1 1/2 hours. Remove the bones and skin from the chicken, returning the meat to the pot.
Heat 3/4 cup of the reserved liquid from the tomatoes in the microwave until just warmed. Stir the flour into the tomato liquid until thick. Add to the stockpot and stir continually until the mixture begins to thicken. Remove from heat and allow to sit for about 15 minutes.
While the chicken mixture rests, bring a large pot of lightly-salted water to a boil. Stir the spaghetti into the water and return to a boil. Cook until the pasta is slightly tender but still firm to the bite, about 12 minutes. Drain. Ladle the chicken mixture over the drained spaghetti to serve.
