Pat the chicken breasts dry and cut into 1" cubes.
Combine chicken pieces, flour, curry powder, and salt in a large airtight container. Put the lid on and shake to coat the chicken pieces in flour.
In a large skillet, heat 2-3 tablespoons oil over medium heat until shimmering. Add the chicken to the skillet in an even layer using tongs so you don't dump excess flour into the pan.
Cook the chicken, stirring and turning them until just barely cooked through. They’ll be golden brown on the outside and some of the light breading will start sticking to the bottom of the pan. This can take 5-7 minutes. Remove the cooked chicken to a plate and set aside. They’ll continue cooking as they rest.
Without cleaning out the skillet, add an additional tablespoon of oil over medium heat. Cook onions and peppers in oil 3-5 minutes until slightly soft. Add curry powder and cook an additional 60 seconds, stirring frequently.
Add tomato sauce and tomato paste and stir to coat the peppers and onions. Keep stirring as the tomato mixture starts to caramelize slightly on the bottom of the pan. Add the chicken stock and bring to a boil, stirring frequently as it reduces slightly, about 3-5 minutes. Taste the sauce and adjust seasoning to taste: add salt, pepper, or more curry powder if needed.
Add the chicken back to the sauce, reduce heat to low, and simmer an additional 10 minutes.
Serve over rice, couscous, or any grain of your choosing. Garnish with fresh cilantro if you like it.