Trim mushrooms and halve if small or quarter if large. Crumble enough Stilton, not including rind, to measure about 1 cup. Force garlic through a garlic press.
In a 9-inch heavy skillet heat butter over moderately high heat until foam subsides and sauté mushrooms, stirring, until golden, about 2 minutes. Stir in garlic and cream and gently boil until slightly thickened, about 4 minutes. Season mixture with salt and pepper and divide between 2 shallow 1-cup gratin dishes. Mushroom mixture may be made 1 day ahead and chilled, covered.
Top mushroom mixture with Stilton and on a baking sheet bake in middle of oven until cheese is melted and bubbling, 5 to 10 minutes.
