My grandmother would call this relish. I would call this sauce. You can call it whatever you'd like, but do try it next time you're going to barbecue burgers, dogs, and/or sausages.
Carb wise....watch your portion. It's all good stuff...but onions and tomatoes have natural sugar which makes their carbs worth noticing in a recipe.
This recipe must be started several hours before you plan to eat it. The time is the hardest part. Dump a can of tomato sauce (at least 16 ounces...larger is fine too) into a sauce pan. Put water in the can about 3/4 full and pour that water (with tomato sauce residue) into the same pot. Add about a 1/4 cup of olive oil to the tomato sauce and water in the sauce pan. Cut up an onion. Cut up a pepper. Add salt and pepper to taste. Cover the pot and put in on low on top of the stove for several hours until the sauce is thicker and the onions and peppers are soft.

Spoon the sauce onto a burger or a hot dog or a piece of your favorite sausage. Delicious!