Preheat the oven to 400°F. Grease a 1.5-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 of 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 mushrooms. Cook, stirring often, until golden and tender, 5-7 minutes.
Remove the skillet from the heat and let it slightly cool. Mix in the steamed green beans, sour cream sauce, and half the cheese.
Transfer the mixture to the prepared baking dish. Top with the remaining cheese and pork rind panko.
Bake until the casserole is heated through and the cheese is melted, for about 20 minutes.
Video
Notes
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.
You can double this recipe and bake it in a 9 X 13-inch baking dish.
The pork panko is optional, and you can use almond meal instead, but I do like its flavor and crunch. You can use store-bought pork panko or make your own by placing pork rinds in a resealable bag and using a rolling pin to crush them. The food processor also works well to crush plain pork rinds into pork "breadcrumbs."
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. The pork rind panko will lose its crispiness, but the casserole still tastes great when gently reheated in the microwave, covered, at 50% power. You can also reheat it in a 350°F oven. I don't recommend freezing this casserole.