An easy recipe from Epicurious involves combining chopped tomatoes with onion, parsley, chiles, olive oil and lemon juice to make a fresh tomato relish. A simple recipe for a cooked tomato relish featuring canned tomatoes, vinegar, sugar and spices is available from MyRecipes.
To prepare a fresh tomato relish according to the recipe from Epicurious, finely dice 1 pound of ripe tomatoes, and toss them with 2/3 cup of finely chopped red onion and 1/4 cup of chopped flat-leaf parsley in a bowl. Add 3 deseeded and finely chopped serrano chiles, along with 1/4 cup of extra-virgin olive oil and 3 tablespoons of freshly squeezed lemon juice. Add salt and pepper to taste. Chill the tomato relish for up to four hours before serving.
To make a cooked tomato relish according to MyRecipes, peel and slice 1 1/2 pounds of onions, and soften them with 1 tablespoon of butter in a saucepan over medium-high heat. Pour in 28 ounces of canned tomatoes, 1 cup of cider vinegar and 1 cup of brown sugar. Stir in 1 teaspoon of pepper and 1/2 teaspoon each of ground allspice, cinnamon and cloves. Increase the heat to the high setting, and boil the mixture for 25 minutes or until it thickens to a desirable consistency. Season the tomato relish with salt, and store it in the refrigerator for up to one week.