Using the Sauté function on an Instant Pot (Normal/Medium) or a large frying pan over medium heat, cook the leeks until they begin to soften.
Add the mushrooms and continue cooking until they release their moisture and start to brown.
Stir in the garlic and cook for 1 minute until fragrant.
In a small bowl, whisk the cornflour with a splash of the vegetable stock to make a smooth slurry.
Add the slurry, remaining vegetable stock and oat cream to the pan. Stir until the sauce thickens.
Add the thyme leaves and Dijon mustard, then season with salt and pepper. Simmer for 2–3 minutes until the filling is creamy and well combined.
Transfer the filling to an oven-safe pie dish that fits inside your air fryer basket.
Cut the puff pastry to fit the dish, lay it over the filling and press the edges down to seal.
Cut a few small steam vents in the centre and brush the pastry with oat milk.
Air Fry at 170°C for 25 minutes, cooking until the pastry is puffed, crisp and deep golden brown. If the pastry browns too quickly, loosely cover with foil for the remaining cooking time.