Over high heat, bring pot of salted water to boil. Add beans. Cover; cook 4-5 minutes. Coat 2-quart baking dish with cooking spray. Drain beans; reserve. Preheat oven to 375 degrees.
In large skillet, melt 1 T. butter over medium heat. Add onions; cook, stirring, 3-4 minutes. Add mushrooms; cook, stirring, until softened, 1-2 minutes. Add wine; cook 2 minutes. Stir in Boursin cheese; cook until melted. Toss with beans; transfer to baking dish.
Melt remaining 2 T. butter. Stir in breadcrumbs and Parmesan; sprinkle over green beans. Cover lightly with foil. Bake 20 minutes. Uncover; bake until golden, 10 minutes.