Cook manicotti tubes according to package directions. Drain and set aside.
Preheat oven to 375°F. Heat olive oil in a large skillet over medium-high heat; sauté mushrooms, onion and garlic 2 minutes. Add spinach and sauté until wilted. Remove from heat and cool slightly. Stir in egg white, cheese and salt and pepper to taste. Stuff spinach mixture into cooked tubes.
Spread 1 cup of the sauce in an 11x7x2-inch baking dish. Place stuffed tubes over sauce. Spoon remaining sauce over tubes. Cover and bake 30 to 35 minutes. Garnish with parsley, if desired.
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