• BRING a large saucepan of water to a boil. Cut a shallow X in bottom of each tomato, then submerge and cook until skin loosens, 1 min. Remove with a slotted spoon and submerge into an ice bath for 30 sec. Drain, then peel off skin. Chop to make 5 cups.
• STIR onion, sugar, water, garlic, pepper flakes, mustard, coriander and salt in same saucepan. Cook over medium, stirring often, until sugar is dissolved, 2 to 3 min.
• ADD tomatoes and bring to a boil. Reduce heat to medium. Gently boil, stirring occasionally, until mixture starts to thicken and reduces to 3 ½ cups, about 40 min.
• MASH tomatoes with a potato masher and reduce heat to medium-low. Continue simmering, stirring often, until reduced to 2 cups, 30 more min. Add vinegar during the last 5 min of cooking. Remove from heat.
• LADLE into clean glass jars, then cover with lids. Let cool completely. Refrigerate for up to 1 month

Variations: 

• Best secret burger sauce: Stir together ½ cup mayo and 2 tbsp tomato jam. Add a pinch of freshly ground pepper. Dollop on your burger.
• Tomato-chili Sauce: Stir together ¼ cup tomato jam, 2 tbsp rice vinegar and 1 tbsp water until combined. Serve with spring rolls or use as a salad dressing.