Preheat oven to 350 degrees F. Line a roasting pan with non-stick aluminum foil.
Place prepared potatoes, rosemary, garlic, red onions, salt, and pepper on the roasting pan. Drizzle with some olive oil (just enough to coat the vegetables); mix well.
Drizzle with balsamic vinegar (at least 1/2 cup but may add additional balsamic vinegar, if prefer).
Bake approximately 1 hour. After 1 hour, check for doneness. At this point, check the taste of the vegetables and drizzle a little more balsamic vinegar over the top just prior to serving. Remove from oven.
Transfer to a serving platter.
Makes 6 servings.
Roasted Potatoes and Red Onions Recipe: https://whatscookingamerica.net/karencalanchini/roastedpoatoes_balsamic.htm