Preheat the oven to 180°C / 160°C fan / gas mark 4.
Cook the pasta according to the packet instructions, then drain and set aside.
Heat the olive oil in a large pan over a medium heat.
Add the onion and cook for 4–5 minutes until softened.
Add the garlic and mushrooms, then cook for another 4–5 minutes until the mushrooms are tender.
Stir in the plain flour and cook for 1 minute.
Gradually add the dairy-free milk, stirring well to keep the sauce smooth.
Stir in the dairy-free cream and Dijon mustard, then season with salt and pepper.
Add the cooked chicken, cooked pasta, and half of the dairy-free grated cheese, then stir until everything is coated in the sauce.
Transfer to an ovenproof dish and sprinkle the remaining dairy-free cheese over the top.
Bake for 10–12 minutes until hot through and lightly golden on top.
Serve warm, topped with fresh parsley if liked.