1. Prepare garlic; let stand 10 minutes.
2. Place mustard seeds, celery seeds, allspice, and peppercorns on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely.
3. 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.
4. 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).