Place a large skillet over medium-high heat and melt the butter with olive oil. Mixing both fats prevents burning while giving the dish rich flavor. Allow the pan to fully heat before adding vegetables for better browning.
Add chopped onions and cook until soft and slightly translucent. This step builds a savory foundation and adds gentle sweetness. Stir occasionally so the onions cook evenly without burning.
Add mushrooms and cook until they release moisture and begin to brown. Allow them to sit briefly between stirring so they caramelize properly. Browning gives mushrooms deep, savory flavor.
Mix in cauliflower florets and cook until slightly crispy on the edges but tender inside. Stir occasionally so pieces cook evenly. Avoid overcrowding to help them brown rather than steam.
Add vegetable stock to the pan to help soften vegetables while keeping flavors concentrated. Let it cook briefly until most liquid evaporates. This step keeps the dish moist without becoming soggy.
Add minced garlic, thyme, and half the parsley. Cook briefly until fragrant so garlic does not burn. Season with salt and pepper to taste and mix thoroughly.
Remove from heat and sprinkle remaining parsley on top. Serve immediately while warm for the best flavor and texture. The dish is best enjoyed fresh from the skillet.