Preheat your oven to 400°F (200°C).
Peel the Tennessee onions and slice them into thin rounds.
In a large mixing bowl, combine olive oil, salt, pepper, and brown sugar.
Add the sliced onions to the bowl and toss until well-coated.
Spread the onions evenly on a baking sheet lined with parchment paper.
Drizzle balsamic vinegar over the onions for added flavor.
Roast in the oven for 25-30 minutes or until golden brown and caramelized.
Remove from the oven and let cool for a few minutes.
Garnish with fresh thyme and croutons, if using.
Serve warm and enjoy your Tennessee onions!