In a large bowl, toss the mushrooms with the lemon juice. Preheat the oven to 300° In a large skillet, melt the butter in 2 tablespoons of the pure olive oil over moderately high heat. When the butter starts to brown, add the mushrooms and season with salt and pepper. Cover and cook until the mushrooms release their liquid, about 3 minutes. Uncover and continue to cook, stirring occasionally, until the liquid has evaporated and the mushrooms are deeply browned, about 8 minutes. Add the soy sauce and cook, stirring, for 2 minutes longer. Transfer the mushrooms to a rimmed baking sheet and keep warm in the oven.