Preheat oven to 400°. Heat a large nonstick skillet over medium-high heat. Add sausage to pan, and sauté for 5 minutes or until browned, stirring to crumble. Remove sausage from pan; drain. Wipe pan with paper towels. Melt butter in pan. Add onion; sauté 4 minutes, stirring occasionally. Add mushrooms; sauté 6 minutes, stirring occasionally. Add potatoes and salt; sauté 5 minutes or until browned, stirring occasionally. Stir in sausage and broth. Remove from heat. Spoon potato mixture into an 11 x 7-inch glass or ceramic baking dish coated with cooking spray; top with cheese. Cover and bake at 400° for 30 minutes. Uncover and bake an additional 15 minutes or until golden. Sprinkle with thyme.