In a small saucepan, combine hot water, vinegar, sugar, and kosher salt. Add red pepper flakes if using. Whisk until the salt and sugar dissolve and bring the mixture to a boil for 2-3 minutes.
Remove the saucepan from heat. Add garlic cloves and 1 bay leaf to the brine to infuse flavor.
Place the cut green tomatoes in a clean jar along with black peppercorns, cumin seeds, mustard seeds, and 1 sprig fresh dill. Carefully pour the warm brine over the tomatoes until fully submerged.