Bring a medium pot of salted water to the boil. Add the cauliflower and potato. Bring back up to the boil over a high heat then turn the heat down to simmer – leave to cook for 8 minutes until the cauliflower and potato are soft. Use a slotted spoon to remove the cauliflower and potato from the water to a large jug or bowl – set aside for later and keep the pot of water on the stove.
Add the pasta to the pot of water and bring back up to the boil (add more water to the pot if needed to cover the pasta). Once boiling, turn the heat down to simmer and leave the pasta to cook for 5 minutes until only just barely cooked on the outside with an uncooked centre. Drain the pasta and rinse it under cold water in a colander. Set aside in the colander for later. Return the pot to the stove.
Add 2 tbsp of olive oil to the pot along with the flour. Stir over a medium heat until smooth and cook for 1 minute. Combine the milk and stock together in a jug – gradually mix this liquid into the flour-oil paste in the pot, stirring in between additions until smooth. Once all the liquid has been added, bring the mixture to a simmer over a medium-low heat and let it cook for 2 minutes – it won’t be very thick right now but don’t worry!
Pour the hot contents of the pot and the tablespoon of mustard into the jug/bowl containing the cauliflower and potato. Blend using a hand blender until completely smooth and thick. Season to taste with salt and black pepper. Set the sauce aside.
Preheat the oven to 180°C (350°F).
In a medium frying pan, heat the remaining 1 tbsp of olive oil over a medium-high heat. Add the mushrooms and let them cook, stirring occasionally until they’ve shrunk down and are releasing liquid (about 5 minutes). Add the garlic, thyme and a pinch of salt to the frying pan, turn the heat down to low and cook for 1 minute. Add the kale to the frying pan and cover with a lid or large plate. Leave to steam for 5-7 minutes until the kale has wilted down. Remove the lid and take off the heat.
In a medium casserole dish or roasting tin, mix the pasta, mushrooms, kale and sauce. Mix the topping ingredients (except the olive oil) in a small bowl and sprinkle over the pasta. Drizzle with some olive oil, slide the dish into the oven and bake for 20-30 minutes until the topping is slightly browned. Divide between bowls and serve hot.