Place onion slices in a medium heatproof bowl; set aside. In a small saucepan warm the white wine vinegar, sugar, and 1/2 teaspoon of the salt over medium heat, stirring to dissolve sugar. Pour over onions; toss to coat. Cover and chill at least 30 minutes or up to 24 hours, stirring occasionally.
For dressing, in a small bowl whisk together the mustard, white balsamic vinegar, pepper, and 1/4 teaspoon salt. Gradually whisk in the olive oil.
Place tomatoes in a large bowl. Add dressing and toss to coat. Top with drained pickled onions and microgreens.