Step 1 Place yolks in large bowl; whisk until blended. Add mushrooms, 3/4 cup (175 mL) cheese, thyme and salt; whisk until combined.
Step 2 Place egg whites in another large bowl; beat with electric mixer until stiff peaks form.
Step 3 Gently fold egg whites into yolk mixture, avoiding over-folding so as not to deflate whites.
Step 4 Spray 10-inch (25 cm) ovenproof skillet with cooking spray. Pour mixture into skillet. Cook over medium heat until bottom is set, about 2 minutes. Sprinkle remaining cheese over top.
Step 5 Place skillet in preheated 425°F (225°C) oven and bake until top is puffed and golden, and frittata is set, 8 or 9 minutes. Cut in wedges and serve immediately, with warmed pasta sauce spooned over each serving, if desired.