Combine mayonnaise and horseradish in bowl. Refrigerate until ready to serve. Combine 2 tablespoons of butter, garlic powder, and Parmesan cheese in another bowl. Set aside until ready to toast buns. Sauté mushrooms in remaining butter until tender. Keep warm. Combine beef, steak sauce, and soup mix. Shape into 4 burgers. Grill over medium-high heat for 4 to 5 minutes per side until burgers are cooked through (internal temperature should reach 160°F). Top burgers with Swiss cheese just before removing from grill. Spread Parmesan cheese mixture on each top bun. Lightly toast on grill. Place burger on bottom bun. Top with horseradish mixture and a spoonful of mushrooms. Sprinkle with onions. Serve open-faced with lettuce leaf and tomato slice on top bun. Serves 4.
Wine Recommendation—Tassel Ridge Iowa Sabrevois
2023 Iowa Sabrevois is a dry red with bold aromas of spices and leather on the nose followed by blackberry and plum notes with hints of baking spice, smoke, and licorice on the palate. Made from Sabrevois grapes grown in our Iowa vineyards, 2023 Iowa Sabrevois pairs well with grilled meat. Serve at room temperature. Store at 55°F.