Fill a large pot (such as: a Dutch oven or soup pot) with water. Place the pot on the stove-top and bring the water to a rolling boil.
Prepare an ice bath for the green beans by filling a large bowl with cold water and a handful of ice cubes.
Boil the green beans for about 5-7 minutes. The beans should be bright green and still have a bite to them. We don't want mushy beans. Immediately drain the beans from the hot water (or use tongs to remove the beans) and plunge the cooked beans into the ice bath.
Allow the beans to rest in the ice bath for 5 minutes and then drain the beans from the water.
Preheat the oven to 375F.
Add 3 tablespoons of butter to a large pot or skillet (I use the same pot used to cook the beans). Add onions and mushrooms and saute for a few minutes (3-4 minutes) to begin to soften. Stir in the minced garlic.
Add the flour to the pot and stir to coat the veggies in the flour. Cook for a couple of minutes to get the flour taste out.
Add the sour cream, mustard, salt, and pepper to the veggies. Once combined, add the cooled green beans and stir until the beans and veggies are well coated.
To make the topping, in a small skillet or saucepan, melt 1 tablespoon of butter. Turn off the heat. Stir in the remaining topping ingredients: panko, parmesan cheese, parsley, and salt.
Grease a small casserole dish (my baking dish is 10.5x 7--like this). Pour the prepared green beans into the dish, pressing them down to level.
Top the casserole with the breadcrumb mixture. Bake for 18-22 minutes, until bubbling and topping begins to brown. Serve warm.
Notes
*You'll need to use a substitute for the flour if you're wanting to make this gluten-free. My recommendation is to use an all-purpose gluten-free flour blend or something that subs well for gluten-based flours, like cassava flour. **Ian's brand makes a gluten-free panko (looks like this) if you're looking to make a gluten-free casserole. This brand does use canola oil, which I typically avoid, but it's the best option I've found for a gluten-free panko. When it comes to buying panko breadcrumbs, read the ingredient list on each option. Stick to a breadcrumb with ingredients you can pronounce and would use in your own kitchen. You may need to visit a health food store (like Whole Foods) or look on Vitacost (like this brand) for such an option. I haven't found great options in the conventional stores in our area.