In a blender, combine tomatoes, onion, garlic, bell pepper, oregano and 1/2 cup of chicken broth. Blend until smooth.
Heat oil in a saucepan over medium heat. Pour in the blended sauce (careful-it might splatter). Simmer for 10-15 minutes, stirring occasionally. Add more water if it thickens too much. Adjust salt to taste. The salsa should be smooth, pourable, and lightly seasoned-just enough to enhance the pupusa.