2 tablespoons (1/4 stick) butter
12 ounces fresh mushrooms, sliced
18 to 20 slices quality white sandwich bread, crusts trimmed
10 ounces ham, cut into 1/2-inch pieces
10 large eggs
3 cups (packed) grated Gruyere cheese (12 ounce)
3 1/2 cups milk (do not use low-fat or non-fat)
2 tablespoons chopped fresh thyme or 2 teaspoons dried thyme
2 teaspoons dry mustard
3/4 teaspoon salt
1/2 teaspoon pepper

Melt butter in heavy medium skillet over medium-high heat.

Add mushrooms and saut until golden, about 10 minutes. Season with salt and pepper.

Generously butter a 13 x 9  x 2-inch glass baking dish. Line bottom with half of bread slices, cutting to fit. Top with ham, then 2 cups of cheese. Top with mushrooms. Add enough remaining bread in single layer to cover.

Whisk eggs, milk, 1 tablespoon thyme, dry mustard, salt and pepper in large bowl to blend. Pour over bread. Cover and refrigerate overnight.

Remove from refrigerator and let sit 30 minutes while preheating oven to 350 degrees F.

Uncover baking dish. Bake until puffed and golden, about one hour.

Mix remaining cup of cheese and 1 tablespoon thyme; sprinkle on top. Bake until cheese melts, about 3 minutes.

Let sit for 10 minutes before serving.

Serves 8 to 10.

Leftovers are equally as good re-warmed the next day in the microwave.