Combine 1/2 cup vinegar, 1/2 cup water, and 1/2 tsp. salt in a medium heatproof glass bowl or mason jar. Stir until the salt dissolves. Place onion in a colander set over the sink. Pour boiling water over the onions to wilt them slightly. Add the onions to the vinegar mixture, pressing to submerge them. Cover with a lid or plastic wrap and set aside.