In a wide, deep skillet over medium heat, warm the oil. Add the onion, lemongrass, garlic, ginger and serrano. Cover and cook, stirring occasionally, for 5 minutes. Add the mushrooms, sweet pepper and zucchini, raise the heat slightly, and cook uncovered, tossing and stirring often, until the vegetables are almost tender, 8 to 10 minutes. Stir in the coconut milk, chicken stock, wine, fish sauce, chili paste and brown sugar (The dish can be prepared to this point up to 1 day in advance. Cool, cover and refrigerate).