01 -
Halve the cherry tomatoes, slice the cucumber into thin rounds, and shave the red onion on a mandoline or with a sharp knife. Pack them into a clean quart jar, alternating colors.
02 -
Combine white vinegar, water, sugar, salt, smashed garlic, peppercorns, and red pepper flakes in a small saucepan. Bring to a gentle simmer over medium heat, stirring until the sugar and salt dissolve completely, about 3 minutes.
03 -
Let the brine cool for 2 to 3 minutes, then pour it over the vegetables. Press down with a chopstick to remove air pockets and ensure everything is submerged. Seal the jar.
04 -
Let the jar cool to room temperature, then refrigerate for at least 2 hours. For best flavor, wait 24 hours before serving.