This side pairs well with seared steak, shrimp, or fish.
In a large skillet, heat 2 tablespoons oil over medium-high heat. Add mushrooms and cook, undisturbed, 2 minutes. Continue to cook, stirring occasionally, until mushrooms are golden brown, about 5 minutes. Add 1 tablespoon oil, garlic, and shallot, and cook, stirring, until shallot is softened, 3 minutes. Season with salt and pepper. Add spinach and cook, stirring, until wilted, 2 minutes. Season to taste with salt and pepper.