Preheat the oven to 500°. In a skillet, toast the coriander, fennel and cumin seeds over high heat, stirring, until the spices are lightly browned, about 2 minutes. Transfer to a plate and let cool. Scrape the spices into a spice grinder, add the salt and pepper and grind to a powder. Transfer to a small bowl and stir in the lemongrass and garlic.