Tomato paste is a sweet, rich concentrate used as a thickener, binding agent or sweetener. If you don't have tomato paste on hand, you can substitute with tomato sauce—all it takes is cooking down the sauce for about 30 minutes to reduce the moisture first.
Heat a saucepan over medium heat on a stove top.
For every 1/2 cup tomato paste called for in the recipe, pour 1 cup tomato sauce into the saucepan.
Stir the tomato sauce with a wooden spoon until it simmers, to prevent it from burning on the bottom of the pan.
For every 1 cup of tomato sauce, add 1 tablespoon white sugar and stir for 1 minute to dissolve.
Let the tomato sauce simmer for approximately 30 minutes, stirring occasionally. The sauce should boil down to approximately half its original volume and begin to thicken.
Remove the saucepan from the stove top and let the thickened tomato sauce come to room temperature; this may take up to an hour.
Stir the tomato sauce to even out its consistency.
Use the thickened sauce just as you would tomato paste.