Heat the oil in a medium saucepan over medium heat. Add the peppers, onions and garlic and cook until soft, about 2 minutes. Add the chili powder, cumin, cayenne, bouillon cube and some salt and pepper and cook until the spices darken and their flavors are released, about 2 minutes. Sprinkle in the flour, stir to combine and cook it for another minute. Add the stock and stir to combine, then allow the liquid to thicken. Add the tomato sauce and 1 1/2 cups water and bring to a boil. Cook until the sauce is nice and thick and reduced by about a third, 5 to 10 minutes.
Allow the sauce to cool slightly, then use an immersion blender to completely puree the sauce until smooth. (Alternatively, you can transfer it to a blender to puree it; just make sure the sauce is cooled, as blending hot liquids is dangerous!)
The sauce will keep in the refrigerator for up to a week and can also be frozen! Just reheat the sauce in a saucepan or skillet for whatever recipe calls for it.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.
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