TO MAKE PICKLED ONIONS: Make one day in advance. Combine ½ cup warm water, ½ cup white vinegar, 1 tsp. granulated sugar and 1 tsp. salt in a large jar and stir until sugar and salt are mostly dissolved. Set aside. Thinly slice 1 small red onion and place in a bowl. Cover with 4 cups of boiling water, let sit 1 minute. Strain onions, and carefully transfer to the jar. Push onions down with a wooden spoon to ensure they’re fully covered in liquid. Allow to cool before covering with a lid. Refrigerate until ready to enjoy.