1. In nonstick skillet, saute chicken in oil over low heat until it's cooked through, stirring constantly (10-12 minutes).
2. Add onion, garlic, bell pepper, and tomato. Cook until vegetables are softened (5-6 minutes). Remove mixture from skillet and cool in the fridge (30 minutes).
3. While chicken cools, place salt and flour in large mixing bowl. Stir in enough water to make stiff, tortilla-like dough.
4. When chicken mixture has cooled, stir in cheese.
5. Divide dough into 24 portions. Roll dough into balls and flatten each into 1/2" thick circle. Put spoonful of chicken mixture in middle of each circle of dough and bring edges to center. Flatten ball of dough again until it is 1/2" thick.
6. Spray a heavy-bottomed skillet with cooking spray and heat on high. Cook pupusas on each side until golden brown (2-3 minutes). Serve hot.