Cooking tomatoes briefly and then straining them creates a thick liquid known as tomato purée. If you don't have purée available, you can make it with canned tomato paste. It takes just a few steps and lets you substitute tomato paste in recipes that call for tomato purée. Neither has any seasonings added.
Place tomato paste in a glass measuring cup. Add enough water to fill to the 1-cup line. Place the water and tomato paste in a saucepan. Whisk the tomato paste and water together in the saucepan, breaking up thick clumps of paste, until you have a smooth purée.