This creamy mushroom gratin recipe / casserole features tons of layered sautéed leeks and wild mushrooms baked in the silkiest cream sauce that is both naturally vegan and gluten free (without flour or butter). For that coveted je ne sais quoi "gratinee" element make sure to serve with a sprinkling of our easy crispy bread crumb topping included below.
Thinly slice the leeks and add them to a large bowl. Cover with cold water and give them a good stir. Allow to sit for a few minutes for all the sand and grit to fall to the bottom.
4 cups leeks
Preheat a large heavy bottom skillet over medium high heat. Using your hands scoop the leeks from the water and add them to the dry hot skillet. Sautee until all moisture has evaporated.
4 cups leeks
Drizzle the leeks lightly with olive oil and season with a pinch of sea salt. Stir well and continue sautéing until the leeks have completely wilted and start to get some color.
Olive oil as needed
Lightly oil a 10 inch round baking dish or line with parchment paper. Spread the cooked leeks in one layer on the bottom of the dish and set aside.
4 cups leeks
Cook the Mushrooms
Rinse and dry the mushrooms on a lint free tea towel. (Depending on the variety used you might want to tear them into smaller pieces, half them or use whole). Add the mushrooms to the same skillet you cooked the leeks in and turn up the heat. Sautee long enough at relatively high heat so all the water released by the mushrooms evaporates (sometimes mushrooms release a lot of water sometime none at all). Stand by!
1 lb wild mushrooms
Push the mushrooms to the sides of the skillet and add a drizzle of olive oil in the center. Add the minced garlic and cook until the fragrant. (Do not burn the garlic.
1 large garlic clove, Olive oil as needed, 1 Tbsp fresh thyme leaves
Stir well then add the white wine and thyme. Continue cooking until the wine has evaporated. Season the mushrooms with a small pinch of salt, remove from heat and transfer to the casserole with the leeks. Spread in an even layer.
1/4 cup dry white wine
Preheat your oven to @375”F
Make the Cashew Cream
Add the cashew pieces, water, nutritional yeast, onion powder and salt to a very powerful blender such as a Vitamix. (If a less powerful blender is used, make sure to soak (or boil) the cashews in advance for 20 minutes in hot water before processing. Blend until creamy smooth.
1 cup raw cashew pieces, 1.5 cups water, 2 Tbsp nutritional yeast, 3 tsp onion powder, 3/4 tsp salt
Pour the cashew cream over the mushrooms and leeks and give it a little shake to evenly distribute. Cover with aluminum foil and transfer to the preheated oven. Bake the casserole for 45 to 50 minutes until the cream has set and thickened. Sprinkle with the crispy bread crumbs and serve hot.
1 lb wild mushrooms, 4 cups leeks
Crispy Bread Crumb Topping
Preheat a 10 inch skillet with 1.5 Tablespoon of olive oil over medium heat. Add the herbs and fry until crispy. Stir in the bread crumbs and toast moving them around until golden brown all over taking care not to burn them. Remove from heat and a dd a pinch of sea salt flakes.
1.5 cup panko breadcrumbs, 1.5 Tbsp olive oil, 1 Tbsp fresh herbs like thyme or sage, 1 pinch sea salt flakes
Plantricious + WFPB
To make the casserole compliant make sure to omit all oil or replace with water or veggie stock as needed.
Notes
Swaps - Instead of leeks you could opt for equal amounts of green onions, thinly sliced fennel bulb, zucchini and even artichoke hearts.
Potatoes - thinly sliced golden potatoes can be used to layer the bottom of the dish. If going this route make sure to use a mandolin on the thinnest setting, you should be able to read the morning paper through them. You could also layer the top before adding the cream sauce, in this case make sure to bake your gratin for a full hour to ensure the potatoes are fully cooked.
Make Ahead - All the ingredients can be prepped ahead and assembled in the casserole but store the Cream separately in the refrigerator until ready to bake.
Leftovers Storage + Reheating - If you happen to have any leftovers at all simply refrigerate in a lidded container up to 5 days. You can quickly microwave to reheat or place the casserole covered in a 350"F oven for 20 to 30 minutes until hot in the center.