This wild mushroom dish has a champagne-vinaigrette pan sauce that can also be drizzled over our Salt-Baked Chicken Breast Stuffed With Asparagus.
In a skillet large enough to hold the mushroom mixture without crowding, melt butter with 1/4 cup oil over medium-high heat. Add onions, and season with 1 teaspoon salt; toss to combine. Cook, stirring often, until onions are translucent, 3 to 5 minutes. Add mushrooms, and toss to combine. Season with salt and pepper. Cook, stirring often, until mushrooms are almost tender, 3 to 4 minutes.
Add scallions. Cover; cook, stirring occasionally, until mushrooms and scallions are tender, 7 to 8 minutes.
Stir in vinegar and remaining 1/2 cup oil. Season with salt and pepper.
If you're serving this dish with Salt-Baked Chicken Breast, drizzle any remaining pan sauce from the mushrooms over the cooked chicken just before serving.