- Cook whole zucchini in boiling salted water until just barely tender, about 5 to 8 minutes.
- Heat the oven to 350 F.
- Cut each zucchini in half lengthwise; scoop out the squash, leaving about 1/4 inch of shell. Mash the squash you took out; set aside. Place the shells in a shallow baking dish; set aside.
- In a heavy skillet over medium heat, cook sausage with chopped onion. Drain off excess fat and stir in mashed zucchini.
- Reserving 2 tablespoons of parmesan cheese for topping, add remaining parmesan cheese to the meat mixture along with the crushed crackers, egg, thyme, and salt and pepper. Mix well.
- Spoon the mixture into the zucchini shells and sprinkle with the reserved 2 tablespoons parmesan cheese.
- Bake for 20 to 30 minutes.