In a wide, shallow bowl, whisk together sugar, pepper, garlic, lemon grass, fish sauce, and 2 tbsp. olive oil. Add beef, toss to coat, and refrigerate 2 hours.
Rub remaining 1 tbsp. oil over onions. Prepare a gas or charcoal grill for medium-high heat (you can hold your hand 1 to 2 in. above grill level only 3 to 4 seconds). Brush grill with vegetable oil. Thread onions onto two 8-in. metal skewers. Lay onion skewers on grill and cook (close lid on gas grill) until softened, about 15 minutes, turning skewers every 5 minutes or so.
When onions are cooked halfway, lay beef slices on grill and cook (close lid on gas grill), turning once, 3 to 5 minutes.
Arrange beef and onions on a platter and garnish with cilantro and peanuts.
Note: Nutritional analysis is per serving.