Wipe the king oysters mushrooms down with a clean cloth to remove any grit. Chop 1 inch of the stalk to remove the tough base. Carefully but firmly drag the prongs of a fork down the length of the mushroom to shred the “meat” (this is easier if you lay the mushroom down.) Break up any bits of the mushrooms that doesn't shred. Set them aside.
Melt the butter and olive oil in a large pan over low to medium heat and add the onion, carrots, celery. Cook for around 10-15 minutes or until the vegetables are tender and then toss in the garlic for another 30 seconds or until fragrant.
Stir in the flour and cook for another 3 minutes, stirring occasionally.
Add the vegetable stock, thyme, and bay leaves and then stir in the milk, tamari/soy sauce, pepper, and sea salt. Bring the mixture up to a simmer.
Stir in the shredded mushrooms and reduce the heat to low. Cook the pie filling for 10 minutes or until the liquid has reduced to a pie gravy consistency. Note. If your pie mixture needs more gravy, you can add an extra cup of stock to your pot. Add it and continue cooking the gravy to your desired consistency. Just make sure to taste the gravy and add more seasoning if needed. Stir in the peas and cook for another minute before removing the pot from the heat to cool while you prepare your pastry. And this stage you might want to remove the bay leaves from the pie filling.