Instructions:
Preheat oven to 375º F.

Lightly butter a 7 x 11 inch baking dish.  

To make topping: Combine ½ cup of cheese with breadcrumbs in a small bowl. Toss with 2 tablespoons melted butter. Set aside.   

To make casserole: Peel potatoes and cut into ½-inch dice. In a 2-quart saucepan, add potatoes and cover with cold water. Add salt. Bring to a boil, reduce heat and simmer until potatoes are barely tender, about 4 minutes. Drain immediately and set aside.   In a medium skillet, cook bacon over medium heat until lightly brown. Transfer bacon to a paper towel to drain. Discard excess bacon fat from skillet. (It is not necessary to clean the skillet as the remaining brown bits and fat will loosen and add flavor to the casserole.) Add remaining butter to skillet and return skillet to stovetop to melt. Stir in onions and garlic and cook for 2 minutes. Add mushrooms and cook until soft, about 3 minutes. Remove pan from heat and stir in sage, pepper, sour cream, remaining 1½ cups of cheese and reserved bacon.

Spread mixture in prepared baking dish.* Sprinkle breadcrumb mixture evenly on top of potatoes.  Bake for 25 minutes, until cheese is bubbling and top is browned. Let casserole sit for 5 minutes before serving.     

* Casserole can be made a day ahead up to this point. Cover tightly with foil and refrigerate. Cover topping and refrigerate. On the day of serving, remove casserole and topping from refrigerator and let sit at room temperature for about ½ hour. Sprinkle top with breadcrumbs just before baking.   

Serves 6 as a side dish.