Lightly coat baby carrots with olive oil and place carrots on a baking sheet. Season with salt and pepper to taste.
Bake in the oven for 20 to 30 minutes until carrots are tender-crisp and lightly browned.
Prepare onion and brown butter sauce: Heat onion and 2 Tbsp butter in large skillet on MEDIUM-HIGH. Cook, stirring, 10-12 min, until onions are golden. Add additional 4 Tbsp butter. Reduce heat to MEDIUM. Cook 4-5 min, until butter is golden-brown. Season to taste with salt and pepper; keep warm.
Add roasted carrots to onion butter sauce; turning gently to coat with sauce.