Preheat oven to 450.
Butter a casserole dish (I used 12x8").
Line dish with the zucchini slices overlapping each other in rows until filled.
Melt remaining butter over medium heat in a skillet.
Add onion, garlic, salt and pepper to the skillet and cook until onion is translucent (about 3 minutes).
Add the heavy cream.
Stirring until it just boiling.
Add 3 ounces of Parmesan cheese to the skillet, stirring until the cheese melts.
Pour the sauce evenly over zucchini.
Tent the dish with foil and place in the oven 20 minutes.
Remove foil, top with the remaining cheese and bake for 5 more minutes.
Remove from oven and serve.