Prepare the ingredients: Remove stems and seeds from the bell peppers and chop them. Chop the onion, tomato, and garlic cloves. Grind the coriander seeds and peppercorns, or pulverize them with a mortar and pestle.
Add all ingredients to a large non-stick frying pan. Cook over medium heat, stirring often until all ingredients are cooked through, blended and soft.
Let the mixture cool for five minutes.
One cupful at a time, run the mixture through a blender set on high speed, until it's smooth and creamy.
Store your sauce in a glass jar in the fridge for immediate use, or freeze in plastic containers for the future.