Position a rack in the center of the oven and preheat to 400 degrees. Line a large, rimmed baking sheet with aluminum foil, and lightly grease the foil with oil.
In a large bowl, toss together the mushrooms, shallot, garlic, lime leaves, if using, and lemongrass, if using. Add the oil and fish sauce and toss thoroughly to coat. Spread the mushrooms in an even layer on the baking sheet – it’s fine if they overlap – and roast for 20 minutes. Using a spatula or tongs, toss the mushrooms lightly and roast for another 10 to 15 minutes, or until they are well browned and crisp.
Make the sauce: Meanwhile, in a small bowl, mix together the lime zest, juice, brown sugar, fish sauce and chile-garlic sauce.
Serve the mushrooms and sauce family-style, with lettuce leaves for scooping and stuffing and herbs and sauce for sprinkling and drizzling on the side.