Preheat oven to 350 degrees. Cook manicotti shells according to package directions. Cool in large bowl of cold water; set aside. Saute onions in 2 tablespoons butter until tender. Combine spinach, onion, mozzarella cheese, sour cream and egg; set aside. Melt 1/4 cup butter in a saucepan, stir in flour, chicken bouillon and pepper. Stir in milk and Half & Half. Bring to a boil, stirring constantly. Boil and stir one minute.
Remove manicotti shells from water. Pat dry with paper towels. Stuff spinach mixture into the manicotti shells. Spread a small amount of sauce on bottom of a buttered 2-quart or 13 x 9 rectangular baking dish. Place rolls in dish. Spoon remaining sauce over roll-ups. Sprinkle with Parmesan cheese. Bake for 30 to 35 minutes or until hot and bubbly.
