Preheat the oven to 400 degrees F. Drizzle 2 tablespoons olive oil on a rimmed baking sheet. Scatter the rosemary leaves and thyme sprigs on the baking sheet. Season the onions with salt and pepper; arrange on top of the herbs, cut-side down.
Remove 4 wide strips zest from the lemons using a vegetable peeler; set aside. Cut each lemon in half and remove any seeds; add to the baking sheet cut-side down. Roast until the onions are tender and the skins are slightly charred, about 1 hour.
Meanwhile, make the vinaigrette: Combine the vegetable oil, vinegar, mustard and 1/2 teaspoon salt in a blender. With the motor running, gradually drizzle in the remaining 6 tablespoons olive oil and 2 tablespoons water until emulsified. Transfer to a small bowl; add the lemon zest and season with salt and pepper. Let steep at least 30 minutes; remove the lemon zest.
Remove and discard the skins from the onions. Arrange the onions, lemons and herbs on a platter and drizzle with the vinaigrette just before serving. (The roasted onions and vinaigrette can be made up to 4 hours ahead. Serve at room temperature.)