Step 1Cut the tomatoes into sixths, then in half. Sprinkle the chunks with 1 teaspoon salt and let sit for 1 hour.
Step 2In a stainless steel pan, add the tomatoes, sugar, vinegar, garlic, onion, pickling spices, cinnamon stick, mustard, pepper, cayenne and lemon zest. Bring to a gentle boil, stirring. When it boils, reduce the heat to medium and simmer for 30 minutes, stirring often. Remove the cinnamon stick.
Step 3Pass everything through a food mill using the finest sieve available. (If you don't have a food mill, pulse in a food processor, then strain.)
Step 4Return the puree to the pan and bring to a gentle simmer. Whisk in the cornstarch slurry; cook for 8 minutes. Add the remaining salt.
Step 5Pour the mixture into a bowl and cool at room temperature. The flavor will mature after the ketchup has been refrigerated overnight.
Variations: Make a sauce to go with smoked meat sandwiches or roasted meats by adding in step 2: 1 tablespoon molasses, one-fourth teaspoon allspice and one-eighth teaspoon ground cloves. For a sauce for duck dishes, add in step 2: the zest of 2 oranges and the juice of 1 orange.