In a large, deep skillet or pot, heat the olive oil over medium heat.
Add the diced chicken to the pot and season with salt and pepper. Cook until the chicken is browned and cooked through, about 5-7 minutes. Remove the chicken from the pot and set aside.
In the same pot, add the chopped onions and sauté until translucent, about 3-4 minutes.
Stir in the minced garlic, diced bell pepper, and zucchini. Cook for another 3-4 minutes until the vegetables are slightly tender.
Sprinkle in the Italian seasoning and red pepper flakes, mixing well.
Add the diced tomatoes (with their juice) and chicken broth to the pot, bringing the mixture to a gentle boil.
Once boiling, add the spaghetti, pressing it down into the liquid if needed. Stir occasionally to prevent sticking.
Reduce the heat to medium-low and simmer for about 10-12 minutes, or until the spaghetti is al dente and most of the liquid has been absorbed.
Return the cooked chicken to the pot and stir to combine everything. Adjust seasoning with additional salt and pepper if desired.
Remove from heat and let sit for a couple of minutes before serving.
Notes
Serve topped with fresh basil and grated Parmesan cheese.