1Preheat oven to 350. Grease a 3 quart rectangular baking dish; set aside. In large pan, cook potatoes in boiling lightly salted water for 5 minutes. Drain and set aside.
2For sauce, in large pan, cook mushrooms, onions, and garlic in hot oil over medium heat til tender. Stir in flour, salt and pepper. Add milk, all at once. Cook and stir til thickened and bubbly. Stir in sage. Remove from heat.
3Layer half the potatoes in prepared dish. Pour in half of the sauce. Sprinkle with ½ cup cheese. Repeat with remaining potatoes and sauce. Cover dish with foil. Cover and chill remaining sauce.
4Bake for 35 minutes. Uncover and sprinkle top of gratin with remaining cheese. Bake for about 35 minutes more or til potatoes are tender and top is golden. Let stand for 10 minutes before serving. Makes 8 side dishes
5Variations: Parmesan Potatoes Au Gratin:
Prepare as directed, except omit mushrooms and sage and substitute shredded Parmesan cheese for blue cheese.
Apple Smoked Cheddar Gratin:
Prepare as directed, except omit mushrooms. Cut 2 large cored Granny Smith apples into thin wedges. Reduce amount of sage in sauce to 2 teaspoons. Put half the apples over each layer of potatoes. Substitute smoked cheddar or Gouda for blue cheese.
To make ahead: Assemble gratin as directed but do not preheat oven. Cover with plastic wrap and chill for up to 24 hours. Bake in a 350 oven for 50 minutes. Uncover and top with remaining cheese. Bake for 40 to 50 minutes more or til potatoes are tender.
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