Peel the sweet potatoes and cut off the ends. Cut potatoes lengthwise into 1-inch wedges. Lightly coat sweet potato wedges with olive oil, spread the potatoes in a single layer on a baking sheet and roast in middle of oven 20 minutes. Turn wedges over with a spatula and roast until tender and slightly golden, 10 to 15 minutes more.
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 sweet potatoes to onion butter sauce; turning gently to coat with sauce.