Cut zucchini in half and dig out the meat with a spoon. You will be creating zucchini boats for the stuffing mixture. Do not over scrape. Just dig out enough of the middle for a boat shape. Save the zucchini meat for the stuffing mixture.
Place the zucchini skin side down in the baking dish.
While waiting for oven, in a medium skillet, cook the onion and sausage. Drain any excess fat off. Make the stuffing mix according to the package. After stuffing is prepared, place in a bowl with sausage, onion, zucchini meat together
Put the stuffing mixture over the zucchini in the baking dish. Pour the diced tomatoes over the stuffed zucchini mixture and cover with foil.
Cook dish for a total of 45 minutes. Halfway through baking, remove dish and cover the zucchini mixture with the feta cheese. Put back in oven uncovered to cook the balance of the way and melt the cheese.