Heat 2 tablespoons butter or olive oil in a skillet over medium-high heat. Sauté the diced onions and sliced cremini mushrooms for 4 to 5 minutes until caramelized and tender. Add the minced garlic and cook 1 minute more until fragrant. Transfer into the slow cooker insert.

Place the meatballs into the slow cooker over the mushroom-onion mixture.

In a medium bowl or measuring pitcher, whisk together the halal beef broth, Worcestershire sauce, Dijon mustard, smoked paprika, dried thyme, kosher salt, and cracked black pepper. Pour the liquid evenly over the meatballs and mushrooms.

Cover with the lid and cook on LOW for 4 to 5 hours (or on HIGH for 2 to 3 hours) until the meatballs are completely tender and thoroughly infused with savory aromatics.

30 minutes before serving, stir the cornstarch slurry into the slow cooker. Cover and cook on HIGH for 15 to 20 minutes until the beef gravy bubbles and thickens noticeably.

Turn the slow cooker to WARM. In a small bowl, whisk 1/2 cup of the hot gravy into the room-temperature sour cream and softened cream cheese to temper. Stir this tempered mixture gently back into the slow cooker until melted into a luxurious, glossy stroganoff sauce.

Ladle generously over warm buttered egg noodles or creamy mashed potatoes. Garnish with fresh chopped Italian parsley and fresh cracked black pepper.