Prepare boiling water canner. Heat jars in simmering water until ready for use. Do not boil. Wash lids in warm soapy water and set aside with bands.

Tie pickling spice and mustard seeds in a square of cheesecloth creating a spice bag. Set aside.

Toss tomatoes, peppers, onions, garlic and salt in a large glass or stainless steel bowl and set aside.

Combine vinegar, water and sugar in a large stainless steel saucepan. Bring to a boil over medium-high heat, stirring to dissolve sugar. Add reserved spice bag, reduce heat and boil gently for 10 minutes, until spices have infused the liquid. Add vegetables and boil gently for 10 minutes, or until vegetables are heated through and slightly tender. Remove from heat. Discard spice bag.

Pack hot vegetables into hot jars leaving 1/2 inch headspace. Ladle hot pickling liquid into jar to cover vegetables leaving 1/2 inch headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot pickling liquid. Wipe rim. Center lid on jar. Apply band until fit is fingertip tight. Place jar in boiling water-canner. Repeat until all jars are filled.

Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner, and bring water to a rolling boil. Process quart jars 15 minutes, adjusting for altitude. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12-24 hours. Check lids for seal, they should not flex when center is pressed.