Heat a large skillet and then add your onion with a little water to prevent sticking. Cook on medium heat for about 2 minutes, and then add your garlic. After 1 minute, add your bell pepper and mushrooms, adding a bit more water if needed, (remember that the veggies will release some water).
Add all your spices and let them incorporate with the veggie mix for a couple of minutes, then add all the tomatoes.
Bring to a simmer and then turn to low heat and simmer, covered for 20 minutes stirring occasionally.