Place onions, butter, and sugar in a pressure cooker or Instant Pot. Sautee for 15-20 minutes or until onions are golden brown and caramelized. Be sure to keep an eye on them so they don't burn.
Add the flour and garlic. Stir for 1 minute or until the flour is cooked through. Add the broth and balsamic vinegar. Seal pressure cooker and set heat to high pressure; Set cooking time to 5 minutes. After the time is done, release pressure by allowing steam to vent, then remove the lid.
To Serve: Preheat broiler and move oven rack to top position. Spoon broth into 4-6 ovenproof serving bowls. Top with a slice of toast and a generous sprinkle of cheese. Set the bowls on a rimmed baking sheet. Broil until cheese is melted and browned in spots. Serve immediately and enjoy!