adapted from http://spanishjourneys.com/oliveme/2011/09/23/tomato-jam-with-museu-de-confitura-seal-of-approval/

makes about 4 half pints

Ingredients
3½ lbs perfectly ripe plum tomatoes
1 apple, peeled, cored, & chopped
1½ lbs sugar
1 oz (two tablespoons) freshly squeezed lemon juice a big pinch of salt
a sprig of fresh thyme

Steps

1. Blanch the tomatoes for several seconds in boiling water, and then shock the tomatoes by submerging them in iced water to stop the cooking. Peel and core the tomatoes and put them into a large, heavy jam-making pot.
2. Add the sugar, lemon juice, salt, and branch of thyme to the tomatoes. Bring to a simmer, then a steady boil, stirring every few minutes. Watch the jam closely as the water cooks off and the juices become syrupy: you’ll need to stir it steadily to make sure it doesn’t stick to the bottom of the pan. Squash any big chunks of tomato while you’re at it. Skim off any foam that forms on the surface (those dense little bubbles will cloud the jam’s sparkle later if not removed). The jam will begin to set up in about 25 to 35 minutes. When it’s softly set, remove the thyme and ladle the jam into sterilized hot jars and seal.

Note: If you like, you can add spices to this jam, such as a pinch of smoky cumin and/or spicy pimentón, a cinnamon stick or a couple of cloves (that you can fish out before putting the jam in the jar), or a bit of ground ginger or cardamom.