Step 1

Heat oven to 400 degrees. FOR THE TOPPING: Pulse bread, butter, and salt in a food processor until coarsely ground.

Step 2

Combine bread mixture and onions in bowl; set aside.

Step 3

FOR THE CASSEROLE: Melt butter in large skillet over medium heat.

Step 4

Add mushrooms, salt, and pepper and cook until mushrooms release their liquid, about 5 minutes.

Step 5

Increase heat to medium-high and cook until liquid has evaporated (about 5 minutes).

Step 6

Add garlic and thyme and cook until fragrant, about 30 seconds.

Step 7

Stir in flour and cook until golden, about 1 minute.

Step 8

Slowly whisk in wine, broth, and cream; bring to boil. Reduce heat to medium and simmer, stirring occasionally, until sauce is thickened, about 10 minutes.

Step 9

Toss green beans with corn starch in a large bowl.

Step 10

Transfer to 13x9-inch greased baking pan. Pour warm mushroom mixture evenly over beans.

Step 11

Cover pan with foil. Bake until sauce is bubbling and beans are tender; about 40 to 45 minutes. Stir beans thoroughly after 25 minutes.

Step 12

Remove foil and spread topping mixture over beans. Bake until golden brown, about 8 minutes. Serve.