I first made these little gems for my husband, who loves anything spicy, on year when we had an especially large crop of tomatoes. Now I have to plant extra tomatoes to make sure I have enough to make this specifically!
Fill two clean, quart canning jars with bite sized green tomatoes. Any variety you prefer is fine, as long as they're green. I used Juliette tomatoes here, but have also used Cherry and Sweet 100, all successfully. Distribute sliced onion, jalapeno slices, red pepper flakes, and minced garlic evenly between the two jars. Wipe the rims clean with a damp paper towel.
On the stovetop, boil the rings and lids for the jars in water for 5 minutes. Meanwhile, in a separate pot, heat distilled white vinegar until boiling. (Be sure you have enough to fill the jars. - Any remaining vinegar can be poured back into the container after it cools.) Pour hot vinegar into pre-packed jars leaving 1/4 inch space at the top of each jar. Secure one lid and ring per jar, and leave on your counter to cool. The lid will make a single 'pop' sound as it cools, letting you know that it sealed properly. For Best results and flavor, let it sit at least a week before eating -- If you can wait that long.