Preheat oven to 200°C / 180°C fan / 390°F. Bring a medium pot of water to the boil, add a generous pinch of salt and add the pasta. Cook for 8–10 minutes until al dente, or according to packet instructions. Drain and set aside.
Meanwhile, place the cauliflower and mushrooms into a 30 x 23 x 6cm / 11 x 9 x 2½" baking dish; drizzle with olive oil, sprinkle with salt and pepper and toss to coat. Bake for 20 minutes, until golden, tender and reduced.
For the creamy sauce, place a medium saucepan over medium–high heat and add the olive oil and flour. Whisk constantly for 3–4 minutes until thickened and bubbling; then slowly add the milk in 3–4 additions, whisking well to remove lumps. Once smooth, thick and creamy, turn off the heat and add the nutritional yeast and mustard. Adjust seasoning to taste.
Once the vegetables are cooked, remove the tray from the oven. Add the cooked pasta, kale and creamy sauce. Stir to combine, then sprinkle with some more nutritional yeast. Return to the oven and bake for 12–15 minutes until golden and bubbling.
