Preheat the oven to 400 degrees F. Grease a 1 ½-quart baking dish.
Mix the sour cream with the Dijon, garlic, salt, pepper, and cayenne pepper. Set aside.
Place the green beans in a microwave-safe bowl. Add ¼ cup water. Cover and microwave until tender-crisp, about 4 minutes. Drain well. Set aside.
Melt the butter in a large deep skillet over medium heat. Add the onions and the mushrooms. Cook, stirring often, until golden and tender, 5-7 minutes.*
Remove from heat and let slightly cool. Mix in the steamed green beans, the sour cream sauce, and half the cheese.
Transfer the mixture to the prepared baking dish. Top with the remaining cheese and with the pork rind panko.
Bake until the casserole is heated through and the cheese is melted, about 20 minutes.
*Make sure to cook the mushrooms until they have released their liquids into the pan and the liquids have evaporated. You don't want them releasing their liquids into the casserole. It's fine to double this recipe and bake it in a 9 X 13-inch baking dish. Even if you end up with leftovers, they taste great, as long as you reheat them very gently.