Place flour in a heavy saucepan over medium heat; gradually add milk, stirring with a whisk until blended. Cook for about 6 to 8 minutes or until thickened. Remove from heat, add cheese, salt and 1/4 tsp pepper, stirring until melted.
Heat oil in a large nonstick skillet over medium heat. Add and saute spinach, mushrooms and remaining pepper until spinach is wilted and mushrooms are tender, about 5 minutes.
In a separate skillet coated with cooking spray, scramble egg whites over medium heat for 3 to 5 minutes. Top English muffin halves evenly with scrambled egg whites and spinach-mushroom mixture, then drizzle with cheese sauce. Serve immediately.
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