Prep Steak: Ensure 1 ½ pounds sirloin steak are cut into 2-inch strips (trim any excess fat or connective tissue and discard). Season the steak with kosher salt and ground black pepper.
Sear Steak: Bring a large, deep cast iron skillet to medium-high heat. Add 1 tablespoon butter and let melt; swirl to coat the pan. Set a clean plate near the stovetop and, in batches, sear the sirloin strips approx. 1 minute per side and then transfer to the clean plate. Be sure not to fully cook the beef, we are just searing both sides. Add additional butter, as needed, during this process to ensure the pan doesn't dry out.
Sauté: Add a splash of the beef broth to help deglaze the pan (scrape up all the bits stuck to the bottom of the pan). Add 1 tablespoon butter along with the ½ yellow onion (minced) and 1 tablespoon garlic. Sauté for 1-2 minutes or until garlic is fragrant and onion has softened a bit. Add 2-3 cups sliced mushrooms and move around until they have softened, approx. 3-4 minutes. Use a slotted spoon and scoop out the mushrooms and place in a bowl; set aside (its okay if there are still some onions in the pan).
Creamy Base: Add remaining 3 tablespoons of butter and let melt. Add 3 tablespoons all-purpose flour and use a whisk to mix together until they form a paste (aka a "roux"). Keeping the pan at medium-high heat, slowly add 1 cup beef broth approx. 1/4 cup at a time continually whisking. Continue adding broth and then ¾ cup unsweetened/unflavored dairy-free creamer in the same manner. Once both are added, let the mixture simmer a bit to continue thickening, whisking as you go.
Combine: Add ¼ cup sour cream, 2 teaspoons worcestershire sauce, 1 teaspoon dijon mustard, and ½ teaspoon lemon zest. Whisk to combine. Add back in the beef (as well as any juices that have settled on the bottom of the plate) and mushrooms. Stir to combine. Taste and add additional salt, as-needed (I usually add in a little).
Serve: Enjoy over egg noodles with some fresh chopped parsley and shaved parmesan.
Notes
*Dairy-Free Creamer: we used Nutpods but there are lots of options out there - just make sure it is unsweetened and unflavored!
Gluten-Free: to make this gluten-free use gluten-free flour and a gluten-free noodle of your choosing for serving.
Dairy-free: to make this dairy-free use a dairy-free butter option, use the dairy-free creamer option and use a dairy-free sour cream.
Storage: store leftovers in an airtight container in the refrigerator for up to 3 days.