Preparation

• 在一个大煎锅中火,将一汤匙. of the oil and 1 Tbs. of the butter. Once the butter has melted, add the onions, and cook, stirring occasionally, until browned and caramelized, adjusting the heat as needed, 35 to 45 minutes. About halfway through cooking time, season with 1/2 tsp. salt and 1/4 tsp. pepper. (Note: If the onions begin to stick as they caramelize, especially near the end of cooking, add a bit of water and use a wooden spoon to scrape up any brown bits sticking to the bottom of the pan.)
• Transfer the onions to a bowl. Increase the heat to medium high, and return the skillet to the stove. Add the remaining 1 Tbs. oil and 1 Tbs. butter. Once the butter has melted, add the mushrooms in a single layer and cook, undisturbed, until golden and caramelized, and the pan looks almost dry, about 7 minutes. Lower the heat to low, and season with 1/4 tsp. salt and 1/4 tsp. pepper. Stir and cook until mostly golden and tender, 3 to 4 minutes more.
• Return the onions to the pan, and add the sherry and pepper flakes. Cook until the sherry evaporates (which will happen very quickly), season to taste with salt and pepper, and set aside to cool to room temperature.
• Stir the crème fraîche into the cooled onion-mushroom mixture, and season to taste with lemon juice. Transfer to a serving bowl. Sprinkle with the parsley, and serve at room temperature with the bread or chips.