Cook fresh spinach until it releases its liquid and drain or squeeze thawed spinach to drain all liquid. Heat thawed and drained spinach in the butter with the onion until onion is tender. Combine egg and all cheeses and seasonings in a mixing bowl. When spinach and onion mixture is cooled, add it to the cheese mixture. Meanwhile, cook pasta shells al dente. Cool by rinsing in cold water; drain. Stuff shells with spinach-ricotta filling. Place in large casserole and cover with lasagna sauce. Cook at 350 degrees oven for 30 minutes.