1. Heat a 12-inch nonstick skillet over medium-high heat. Coat pan with cooking spray. Add potatoes to pan; sauté 10 minutes, stirring occasionally. Stir in mushrooms and 1/4 teaspoon salt; sauté 8 minutes or until potatoes are tender, stirring occasionally. Remove from pan; set aside.
2. Wipe pan clean with paper towels. Melt 2 teaspoons butter in skillet over medium-high heat. Combine eggs, remaining 1/4 teaspoon salt, and pepper in a bowl, stirring with a whisk until eggs are frothy. Pour half of egg mixture into pan, and stir briskly with a heatproof spatula for about 10 seconds or until egg starts to thicken. Carefully loosen set edges of omelet with spatula, tipping the pan to pour uncooked egg to the sides. Continue this procedure for about 10 to 15 seconds or until almost no runny egg remains.
3. Remove pan from heat; arrange half of potato mixture, 1/2 cup spinach, and 1/4 cup cheese over omelet in pan. Run spatula around edges and under omelet to loosen it from pan. Fold omelet in half. Slide omelet from pan onto a platter. Cut in half crosswise. Repeat procedure with remaining butter, egg mixture, potato mixture, spinach, and cheese.