Scrub squash and trim off ends; do not peel.
Cook whole squash, covered, in small amount of boiling salted water till just tender, 15 minutes.
Drain well, reserving 1/2 cup liquid; add milk to equal 3/4 cup and set aside.
Chop zucchini coarsely (about 5 cups).
In skillet cook sausage and onion together over medium heat till sausage is browned and onion is tender.
Drain off excess fat.
Stir in squash, milk mixture, crackers, 2 tablespoons of the cheese, eggs, thyme, garlic salt, 3/4 teaspoon salt, and dash pepper; mix well.
Turn mixture into ungreased 10x6x2 inch baking dish; sprinkle with the remaining Parmesan cheese.
Bake at 350° till set and delicately browned, about 45 minutes.