Spinach Gratin

2 tbsp olive oil
5 cups sliced mushrooms (portabellos are recommended, but you can use a mixture of wild varieties, or ordinary button mushrooms)
1 large shallot, minced finely
8 ounces pre-washed baby spinach (or use frozen, thawed, with excess water squeezed out)
1/2 tsp dried thyme
pinch of freshly ground nutmeg
salt and freshly ground pepper, to taste
1 tbsp unsalted butter
1 tbsp flour
3/4 cup good quality vegetable stock
1/2 cup cream
one 18 ounce log of prepared polenta, cut in 1/4 inch slices, or 4 servings polenta, cooked according to package directions
1 cup shredded Gruyere cheese

Preheat oven to 350 degrees. In a large heavy bottomed skillet, heat the olive oil over medium heat and add the mushrooms. Saute, stirring occasionally, until they begin to brown lightly, about 6 minutes. Turn down the heat to medium low, add the shallot, and cook 3 minutes. Turn the heat up to high and add the spinach, thyme, and nutmet, cooking till spinach has wilted, about 2 minutes, stirring frequently. Season to taste with salt and freshly ground pepper. Spread spinach-mushroom mixture evenly in a 2 quart gratin or baking dish. Melt the butter over medium high heat in a small saucepan. Whisk in flour, then add vegetable stock and cream. Cook, whisking, until thickened, about 5 minutes. Season to taste and pour cream sauce over spinach mixture in baking dish. Arrange polenta slices or cooked polenta evenly over spinach, and sprinkle with Gruyere. Cover baking dish with foil and bake 40 minutes in preheated oven. Preheat broiler, remove foil, and broil about 6 inches from heat for 2 minutes, until top is golden. Allow to stand 10 minutes before serving. Serves 8.
(the unbaked gratin can be refrigerated overnight, before baking)