
Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
Bring a large pot of water to a rapid boil. Generously salt, add egg noodles, and cook for about 5 minutes, until they are just al dente, stirring regularly. Drain well and set aside in a large bowl.
While the egg noodles are cooking, heat olive oil in a large skillet set over medium heat. Add ground beef, breaking it up as you do, and cook until beef is no longer pink.
Add the mushrooms to the pan and toss to coat in the fat. Add the shallot and the garlic, stir, and season with salt and pepper. Cook for about 8 minutes, until the mushrooms have softened and are just beginning to brown.
Add the butter to the pan, let butter melt, then add flour and whisk to thoroughly combine. Cook for 2 minutes to let the raw flour cook out. Slowly add in the mushroom stock, whisking as you do to break up any clumps. Whisk in the milk, add the nutmeg, and season with salt and pepper. Let the mixture cook for another 5 minutes or so, just until it has thickened slightly.
Add the sour cream and Dijon mustard to the bowl with the other ingredients and stir thoroughly. Taste, and adjust seasoning if necessary. Add the cooked noddles and stir until well combined.
Pour the mixture into a 9" x 13" casserole dish, cover with foil. If you're going to use this as a make-ahead meal and want to freeze it, now is the time to do so. Before cooking, let the entire casserole come to room temperature than proceed to next step.
Place into the preheated oven. Bake for 45 minutes or until the mixture is hot and bubbling throughout.