Melt 3 tablespoons butter in heavy large skillet over medium heat. Add choppped onion and saute until slightly softened, about 4 minutes. Add chopped spinach and saute 5 minutes. Add corn and saute until vegetables are tender, about 4 mintues. (Can be made 1 day ahead. Cover and chill.) Preheat oven to 350#161#F. Lightly butter 13x9x2-inch glass baking dish. Cook macaroni in large saucepan of boiling salted water until al dente. Drain pasta. Transfer to prepared dish. Mix in cubed cheese.
Whisk flour, salt, mustard, black pepper, cayenne pepper and nutmeg in medium bowl until no lumps remain. Gradually whisk in half and half then whipping cream and sour cream. Add eggs and Worcestershire sauce; whisk to blend. Pour over macaroni mixture; stir to blend. Sprinkle grated cheese over.
Bake macaroni and cheese until just set around the edges but sauce is still liquid in center, about 25 minutes. Remove from oven; let stand 10 minutes to thicken slightly (sauce will be creamy).