Preheat oven to 375°. Place potatoes in a large saucepan; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, 8-12 minutes or until tender. Drain. Meanwhile, in another saucepan, heat butter over medium-high heat. Add mushrooms and onion; cook and stir 6-8 minutes or until tender. Stir in garlic; cook 1 minute longer. In a small bowl, whisk flour, broth and seasonings until smooth; stir into mushroom mixture. Bring to a boil, stirring constantly; cook and stir 1-2 minutes or until sauce is thickened. Remove from heat; stir in sour cream. Arrange half of the potatoes in a greased 1-1/2-qt. or 8-in. square baking dish; top with spinach. Spread half of the hot mushroom sauce over top; sprinkle with 1 cup cheese. Layer with remaining potatoes, sauce and cheese. Bake, uncovered, 12-15 minutes or until heated through and cheese is melted. Let stand 10 minutes before serving