In a large skillet, melt 3 tablespoons of butter over medium heat. Add the onions, sugar and thyme. Season with salt and pepper. Cook until the onions are tender and golden brown, stirring frequently, about 35 minutes. Transfer ½ cup of the onions to a blender; transfer the remaining onions to a bowl. Add the broth and ½ cup cheese to the onions in the blender. Puree until smooth.
PREPARE THE VEAL:
In a large skillet, melt 1 tablespoon of butter over medium-high heat. Season the veal with salt and pepper. Add the veal to the skillet and cook to desired doneness, about 2 minutes per side for medium-rare. Transfer the veal to plates; keep warm.
FINISH THE SAUCE:
Add vinegar to the skillet; bring to a boil, scraping up any browned bits. Add the sauce from the blender to the skillet and cook until heated through. Whisk in the remaining 1 tablespoon butter. Season with salt and pepper.
ONION- CURRANT GARNISH:
In a small saucepan, combine the currants and reserved onions and cook over medium-low heat until heated through.
SERVE:
Set veal on a serving platter. Spoon the sauce over the veal. Top with the onion-currant mixture and sprinkle with the remaining ½ cup of cheese.