This is inspired by cathy's ketchup but that is too thick and apple-y for my tastes. so here's my version.
1 6-oz can tomato paste
1/2 16-oz can tomatoes, drained mostly (I use the rest of the can to make burgers with)
1 Tbsp high-quality balsamic vinegar
1/4 cup apple cider or juice
blend in high-speed blender until smooth. I pour into 4 bowls to freeze. one bowl lasts two meals for me, allowing for a generous sugar-free, salt-free portion in each.