Preheat the oven to 425°F with a rack in the center position.In a large bowl, whisk together the oil, vinegar, lemon juice, mustard, salt, pepper, garlic powder, onion powder, thyme, and brown sugar.Add the onions to the bowl with dressing and toss until wedges are separated and well coated
Using a slotted spoon, transfer the onions to a rimmed sheet pan, reserving the leftover dressing in the bowl
Roast the onions, stirring every 10 minutes, until tender and lightly charred, about 35 minutes
Drizzle the onions with the reserved dressing and toss to combine
Season with salt