Heat oven to 400°F for nonstick sheet pan (425°F for all other pan types). Place oven rack in low position. Melt butter in 10-inch nonstick skillet over medium heat. Add onion and salt; cook 3 to 5 minutes, stirring occasionally, until onion softens. Add mushrooms; cook 5 to 7 minutes or until softened and beginning to brown. Stir in garlic; remove from heat.