Pack your jars. Stuff your veggies tightly into clean mason jars along with the garlic and peppercorns.
Make the brine. Combine vinegar, water, salt, sugar, and red pepper flakes in a small saucepan. Bring to a simmer, stirring until salt and sugar dissolve. Remove from heat.
Pour. Carefully pour the hot brine over the veggies, making sure everything is submerged. Add a jalapeño slice or two on top.
Cool & refrigerate. Let the jars cool to room temp (about 30–45 min), then seal and refrigerate.