In a large pot place peeled onion, tomatoes, and peppers. Bring to a boil.
Allow to boil until tomatoes start to crack.
Remove the onion and the peppers to a blender. Carefully remove the skin from the tomatoes and place in the blender.
In the blender you should have everything from the pot that you boiled the vegetables (not the water) and place the cumin, salt, cilantro, and lime juice.
Blend until combined. Refrigerate until ready to use.