Preheat oven to 400 degrees. Bring a medium pot of water to a boil with a large pinch of salt. Halve, peel, and thinly slice half the onion. Mince the garlic and parsley. Thinly slice the mushrooms.

Grate half the remaining onion into a medium bowl. Then, mix in the ground beef, panko, 1/4 teaspoon nutmeg, 1/4 teaspoon allspice, half the garlic, and half the parsley. Season with a large pinch salt and pepper. Roll into meatballs about 1-inch in a diameter.

Place meatballs on one side of a baking sheet. On the other side, toss mushrooms with 1 tablespoon oil and season with salt and pepper. Place in the oven for 15-20 minutes, until meatballs are cooked through and mushrooms are browned.

Meanwhile, add the egg noodles to the boiling water and cook for 8-10 minutes, until al dente.

While the noodles cook, heat 1/2 tablespoon butter in a large pan over medium heat. Add the sliced onion and cook, tossing, for about 5 minutes, until softened. Add the remaining garlic and cook for 30 seconds, until fragrant. Add stock concentrate and 1/2 cup water. Simmer until reduced by half. Stir in the sour cream and cook for 2 more minutes, until sauce has thickened slightly.

Once the noodles are ready, drain, reserving 1/4 cup pasta water. Add egg noodles, meatballs, and roasted mushrooms to the pan. Toss until combined. Add a splash of pasta water if the sauce becomes too thick. Taste and season with salt and pepper.

Serve with a sprinkle of remaining parsley and enjoy!