For the topping: Pulse bread, butter, and salt in a food processor until coarsely ground.  Combine bread mixture and fried onions, transfer to a freezer bag, and freeze.

For the casserole: Melt butter in large skillet over medium heat.  Add mushrooms, salt, and pepper; cook until mushrooms release their liquid, about 5 minutes.  Increase heat to medium-high and cook until liquid has evaporated, about 5 minutes.  Add garlic and thyme; cook until fragrant, about 30 seconds.  Stir in flour and cook until golden, about 1 minute.  Slowly whisk in wine, broth, and cream; bring to a boil.  Reduce heat to medium and simmer, stirring occasionally, until sauce is thickened, about 10 minutes.

Toss green beans with cornstarch in a large bowl and transfer to a metal baking dish.  Pour warm mushroom mixture evenly over the beans.  Let cool completely, wrap with plastic, cover with foil, and freeze.

To serve: Adjust oven rack to middle position and heat oven to 400F/200C.  Remove plastic from baking dish and replace the foil.  Bake until the sauce is bubbling and beans are tender, about 80 minutes, stirring beans thoroughly after 50 minutes.  Remove foil and spread topping mixture over the beans.  Bake until golden brown, about 8 minutes.  Serve.