Preheat oven to 425°. Spritz mushrooms with cooking spray; place in a 15x10x1-in. pan, stem side up. Sprinkle with garlic salt and pepper. Bake, uncovered, until tender, about 10 minutes.
Meanwhile, in a nonstick skillet, heat oil over medium-high heat; saute onion until tender. Stir in spinach until wilted.
Whisk together eggs and salt; add to skillet. Cook and stir until eggs are thickened and no liquid egg remains; spoon onto mushrooms. Sprinkle with cheese and, if desired, basil.
Health Tip: At about 30 calories and 4 grams of carbohydrates per cap, portobello mushrooms are a low-cal, low-carb way to serve up eggs. Try filling with other combos like cooked sausage, sauce and mozzarella; feta, basil and tomatoes; quinoa, garbanzos and goat cheese.