Heat butter in large skillet over medium heat. Add mushrooms; sauté until golden brown (5-7 minutes), stirring occasionally. Add garlic; cook 1-2 minutes. Stir in cream; bring to a boil; cook until thickened (3-4 minutes). Season with freshly ground pepper to taste.
Preheat oven to 425F.
Transfer mushroom mixture to 1½ quart baking dish sprayed with cooking spray. (Mushroom mixture can be made day ahead and refrigerated, covered.
Sprinkle stilton over mushrooms and bake until cheese is melted and bubbly (5-10 minutes).
While mushrooms bake, slice ciabatta bread in half lengthwise and then cut crosswise into 2 inch pieces.