Preheat the slow cooker while you prepare the rest of the ingredients.
Spread half the onions and mushrooms on the bottom of a slow cooker, followed by the steaks. Sprinkle the steaks with the onion soup mix, flipping them over to coat both sides with the seasoning.
Spread the remaining onions and mushrooms on top, then pour in the beef broth.
Cover and cook on low for 8 hours. Remove about 1/4 cup of the juices from the crock pot. Whisk in the cornstarch to form a slurry. Stir the slurry back into the slow cooker, turn the heat up to high, and continue to cook uncovered, stirring occasionally, another 15-20 minutes, or until juices are thickened. Season to taste with salt and pepper.
Notes
Before doing anything else, turn your slow cooker onto high! Preheating the slow cooker before you add any ingredients to it cuts down on cooking time and allows all your flavorful ingredients to heat up faster.