Preheat oven to 425°. Brush both sides of polenta slices with 1 tablespoon oil; place on a 15x10x1-in. baking pan. Bake 20-25 minutes on each side or until crisp.
Meanwhile, in large skillet, heat remaining oil over medium-high heat. Add mushrooms and shallot; cook and stir 12-14 minutes or until liquid has evaporated and mushrooms start to brown. Add sherry; cook, stirring to loosen browned bits from pan.
In a small bowl, mix flour, salt and pepper; stir in broth until smooth. Stir into pan. Cook and stir 1-2 minutes or until thickened. Add spinach; cook and stir over medium-low heat until slightly wilted, about 2 minutes.
Arrange polenta on a platter; top with mushroom mixture.