Preheat oven to 400F.
Bring water to boiling in lg saucepan or Dutch oven with 1/2 tsp salt. Add squash. Boil until soft, approx. 3-6 min, stirring occasionally. Drain thoroughly. Return to pot. Season with 1/2 tsp salt.
Blend in butter. Blend in sour cream. Blend in mozzarella. Blend in Cheddar. Blend in Gruyere. Blend in 1/2 of Parmesan. Blend in mustard, fresh herbs, oregano, onion powder, remaining salt and pepper.
Transfer to casserole dish. Cover with remaining Parmesan. Bake at 400F until golden and bubbly, approx. 25-35 min.