Coat a large slow cooker with nonstick cooking spray.
In a large, deep skillet over medium heat, melt the butter.
Add the onions and the mushrooms to the melted butter and sauté, stirring frequently, until they are tender, about 5-7 minutes. If needed, reduce the heat to prevent burning.
Add the minced garlic, the garlic powder, the onion powder, chicken bouillon powder, the parsley, the thyme, the salt, and the black pepper to the onion-mushroom mixture and cook, stirring constantly, until it is fragrant, about 1 minute.
Sprinkle the flour over the onion mixture and cook, while whisking, until a thick dough forms and the raw flour taste is gone, about 1 minute.
Gradually add the milk to the sauce, while whisking, and cook until it has thickened.
Reduce the heat to medium-low. Gradually add the chicken broth to the sauce, while whisking, and cook until it has thickened.
Add the Dijon mustard to the sauce and cook, stirring constantly, until it is heated through, about 30 seconds-1 minute.
Transfer the skillet from the heat. Add the sour cream to the sauce and stir to combine.
Pour the sauce into the prepared slow cooker.
Add the hash browns and the sharp cheddar cheese to the sauce and stir to combine.
Spread the hash brown casserole into an even layer and wipe the sides of the slow cooker clean to prevent burning.
Place a few sheets of paper towels between the lid and the slow cooker, then cover and cook the hash brown casserole on high heat, about 2-3 hours. Be sure the paper towels are not touching the hash brown casserole.
About 15 minutes before the hash brown casserole is finished cooking, turn the oven on the broil setting.
Taste the hash brown casserole and season with the salt and the black pepper, as needed. Stir to combine.
Carefully lift the ceramic pot out of the slow cooker and place it in the oven to broil until the top and the edges of the hash brown casserole are crispy, about 2-3 minutes.
Serve the hash brown casserole garnished with the bacon and the chives.