ごちそうをしっかり召し上がれ。

feast; treating (someone); to treat (someone, e.g. to a meal)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

tightly (holding on); firmly; securely; strongly (built); solidly; sturdily; steadily; properly; well; sufficiently; hard (working, etc.); fully; completely; reliably; dependably; levelheadedly; shrewdly; wisely; cleverly

to eat; to drink