Preheat your oven to 425°F. Toss potato wedges with olive oil and seasoning salt on a baking sheet. Roast for 45 minutes until golden and crispy.
While potatoes cook, heat 1 tablespoon olive oil in a medium skillet over medium heat. Add sliced mushrooms and onion, cooking until softened and lightly caramelized, about 8 minutes. Season with salt and pepper, then set aside. (Use a sharp knife to slice the mushrooms evenly — this ensures uniform cooking)
Form the ground sirloin into 4 evenly sized patties. Season both sides generously with salt and press the cracked black pepper firmly into the surface.
Pour the red wine into a shallow dish. Heat 1 tablespoon olive oil in another medium skillet over medium-high heat. Dip each patty briefly in the wine, then sear for 4 minutes per side for medium doneness.
Transfer patties to a plate and top with the mushroom mixture. Cover with foil to keep warm.
Add any remaining wine to the burger skillet and cook over medium heat until reduced to a syrupy consistency, about 3 minutes. Whisk in butter, then flour, then gradually add beef broth and Worcestershire sauce. Simmer until thickened.
In the mushroom skillet, heat the remaining 2 tablespoons olive oil over medium heat. Add spinach and cook just until wilted. Remove from heat and gently fold in crumbled Gorgonzola.
For the sauce, combine sour cream, applesauce, horseradish, and breadcrumbs in a small bowl.
Serve the beef patties with wine reduction spooned over top, alongside the Gorgonzola spinach, roasted potato wedges, and horseradish sauce for dipping.
If you prefer to omit the horseradish, try mixing the sour cream with a touch of Dijon mustard and fresh chives instead.
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