Preheat oven to 400°F.
Melt butter in 9×13-inch baking pan in the preheating oven (this takes 3 to 5 minutes, so watch carefully.)
Remove pan from oven and stir in garlic, onions, rosemary and thyme. Roll potatoes in seasoned butter and place potatoes with skin side up in pan.
Roast 15 to 20 minutes until potatoes are slightly tender. Stir the potatoes to recoat with buttered mixture and continue cooking 10 to 15 minutes.
Drizzle balsamic vinegar over potatoes, season with salt and pepper and return to oven for an additional 5 to 7 minutes. Serve warm.