Place mustard seeds, celery seeds, allspice, and peppercorns on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely.
Combine cheesecloth bag, garlic, tomatoes, and next 3 ingredients (through vinegar) in a large Dutch oven; bring to a boil. Cover, reduce heat, and simmer 20 minutes. Remove cheesecloth bag, and set aside.
Place half of tomato mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Strain smooth mixture through a fine mesh sieve back into pan; discard solids. Repeat procedure with remaining cooked tomato mixture. Add cheesecloth bag, sugar, and salt to pan, and bring to a boil. Reduce heat, and simmer, uncovered, until reduced to 1 cup (about 45 minutes).
This is a great change from the store-bought bottled stuff with loads of HFCS. I didn't have fresh tomatoes on-hand so I used 2 cans of Muir Glen organic tomato sauce. Also, rather than steep whole spices I ground them up in a spice grinder and added them directly to the sauce. I ended up with a greater volume (2 1/2 c?) once it seemed cooked down sufficiently, but the flavor seemed about right. One additional tablespoon of sugar (granulated honey rather) made it perfect. I used some immediately in CL Plum Barbecue Sauce, and the rest I funneled it into two empty 8-oz. plastic soda bottles for my own "squeeze bottles". Burgers, fries...look out!
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