Prepare the Beef: Trim any excess fat from the beef and slice it across the grain into thin strips, about 1/4-inch thick. Season with salt and pepper.
1.5 pounds beef sirloin or ribeye, 1 teaspoon salt, 1/2 teaspoon black pepper
Sear the Beef: Heat olive oil in a large skillet over medium-high heat. Working in batches, add the beef strips and sear for 1-2 minutes per side until browned but still slightly pink inside. Transfer to a plate and set aside.
1.5 pounds beef sirloin or ribeye, 2 tablespoons olive oil
Sauté the Onions: In the same skillet, melt 2 tablespoons of butter. Add the diced onion and sauté until translucent, about 3-4 minutes.
4 tablespoons butter, 1 medium onion
Cook the Mushrooms: Add the sliced mushrooms to the skillet and cook until they've released their moisture and begun to brown, about 5-7 minutes.
8 ounces mushrooms
Add the Garlic: Stir in the minced garlic and cook for another 30 seconds until fragrant.
3 cloves garlic
Add the Flour: Sprinkle flour over the mushrooms and onions, stirring constantly for 1-2 minutes to cook the flour.
2 tablespoons all-purpose flour
Deglaze the Pan: If using, add the dry white wine to deglaze the pan, scraping up any browned bits from the bottom with a wooden spoon.
1/2 cup dry white wine
Add the Broth: Pour in the beef broth and Worcestershire sauce, stirring to combine. Bring to a simmer and cook until the sauce begins to thicken, about 5 minutes.
2 cups beef broth, 1 tablespoon Worcestershire sauce
Cook the Potatoes: While the sauce simmers, place potato chunks in a medium pot, cover with cold water, and add 1 teaspoon salt. Bring to a boil, then reduce heat and simmer until fork-tender, about 15-20 minutes.
2 pounds Yukon Gold potatoes, 1 teaspoon salt
Drain the Potatoes: Drain the potatoes thoroughly and return them to the hot pot for 1 minute to evaporate excess moisture.
2 pounds Yukon Gold potatoes
Mash the Potatoes: Add butter and warm milk to the potatoes, then mash until smooth and creamy. Season with salt and pepper to taste.
1 teaspoon salt, 4 tablespoons butter, 1/2 cup warm milk, Salt and pepper to taste, 2 pounds Yukon Gold potatoes
Add Sour Cream: Reduce the heat under the stroganoff sauce to low. Stir in the sour cream until fully incorporated and heated through.
3/4 cup sour cream
Return the Beef: Return the beef and any accumulated juices to the skillet, gently stirring to combine. Let simmer for 2-3 minutes to warm the beef through.
1.5 pounds beef sirloin or ribeye
Season the Dish: Taste and season with additional salt and pepper as needed.
1/2 teaspoon black pepper, 1 teaspoon salt
Serve: Serve the beef stroganoff over the mashed potatoes and garnish with fresh chopped parsley.
2 tablespoons fresh parsley