For mushroom mix:
2 lbs. oyster mushrooms, cleaned
1 lb. cremini mushrooms, cleaned with stems removed but reserved
6 shallots brunoise
2 carrots brunoise
4 ribs celery brunoise
1 lb. English peas
6 oz. vegan butter
6 oz. gluten-free flour
3 qts. mushroom stock
1 tbsp. rosemary, chopped
For potato mash:
1 head cauliflower, cut into florets
8 Yukon gold potatoes, peeled and diced
1 qt. cashew milk
Salt to taste
For mushroom stock:
2 ribs celery
1/2 bunch thyme
4 qts. water
Clean all mushrooms and reserve the scraps. Roast all mushrooms in a rondo with olive oil until golden brown, then drain on a paper towel–lined tray. In the same pan, roast the mushroom scraps until well charred. Add remaining vegetables and herbs from stock recipe and roast until well charred, then add the water and reduce to 3 quarts and strain through fine mesh strainer.
In a separate large pot, on medium high heat, add vegan butter, shallots, carrots, and celery and cook until translucent. Add flour and cook on low heat for 3-4 minutes, then add mushrooms, rosemary, and hot mushroom stock, whisking constantly until the mixture comes to a simmer and begins to thicken. Add peas and season with salt.
In a separate pot, boil potatoes and cauliflower in lightly salted water until tender. Strain the potato and cauliflower and place in a mixer with cashew milk; whip together until creamy.
Put the mushroom filing into 9 x 16 Pyrex baking dish and use a large spoon or rubber spatula to spread the potato mixture over the top until the filing is covered. Bake at 425 degrees until the sides are bubbling and the potatoes begin to brown.