Wash and slice. Wash the tomatoes, trim the stem ends, and slice them 1/4 inch thick. Halve any very small tomatoes instead of slicing them.
Pack the jars. Divide the tomato slices between 2 clean pint jars, tucking the garlic, mustard seed, peppercorns, dill seed, and pepper flakes, if using, in among the slices as you go. Pack snugly but do not crush the slices.
Make the brine. Combine the vinegar, water, sugar, and salt in a small saucepan. Bring to a boil over medium-high heat, stirring, until the sugar and salt dissolve completely, about 5 minutes.
Pour it hot. Pour the hot brine over the tomatoes, dividing it evenly between the jars, until the slices are fully covered. Leave about 1/2 inch of headspace at the top of each jar.
Seal and cool. Tap each jar gently to release trapped air bubbles, wipe the rims, and screw the lids on. Let the jars cool to room temperature, about 1 hour, then refrigerate.
Wait a day. Refrigerate for at least 24 hours before eating so the brine seasons the tomatoes all the way through. The flavor keeps improving over the first week.
Serve and store. Lift tomatoes out with a clean fork and serve as a condiment. Keep refrigerated and use within 3 to 4 weeks.
