Set grill temperature to 225˚F. Throw in an Apple wood chunk.
Melt the butter in a small saucepan, stir in the garlic and cook over low heat until garlic is just tender, about 10 minutes. Remove from the heat and mix in the lemon juice, salt, and black pepper.
Arrange the onions in a medium-size baking dish. Spread asparagus spears over the onions. Drizzle the garlic and butter mixture of the asparagus.
Place the uncovered baking dish in the grill. Smoke until the asparagus is tender, about 1½ to 2 hours.