Place chicken in a large bowl. Add seasonings and toss to coat.
Heat oil in a dutch oven or other large pot set over medium-high heat. When the oil is hot, add the chicken. Cook until golden brown, about 1-2 minutes per side. Add the onion and garlic and cook until just softened, about 5 minutes. Add the peppers and cook another 5 minutes.
Add the tomatoes and chicken broth. Bring to a simmer; reduce heat to medium and cook 30 minutes.
Whisk together the milk and cornstarch. Stir into the sauce and simmer 10 minutes.
Serve sauce over pasta.