Heat the butter and oil in a large pan or skillet over medium-high heat. Sauté the onion until softened (about 3 minutes).
Add the mushrooms and cook for about 3-4 minutes, then add cauliflower florets. Cook until golden and crispy on the edges, about 8-10 minutes. Veggies must be well browned.
Pour in the vegetable stock and cook for 2 minutes, to reduce the sauce slightly.
Add thyme, 1 tablespoon of parsley and garlic. Cook for half a minute, until fragrant.
Season generously with salt and pepper, to taste, sprinkle with remaining parsley and serve.