clock clock iconcutlery cutlery iconflag flag iconfolder folder iconinstagram instagram iconpinterest pinterest iconfacebook facebook iconprint print iconsquares squares iconheart heart iconheart solid heart solid icon

Mushroom-Basil Au Gratin

  • Prep Time: 35 mins
  • Cook Time: 35 mins
  • Total Time: 1 hour 10 mins
  • Yield: 6 to 8 1x


  • For the casserole:
  • pounds russet potatoes (2 to 3 medium), peeled and thinly sliced (see step 2)
  • 1 medium yellow or white onion, halved and thinly sliced (about 1½ cups)
  • ½ pound medium white or cremini mushrooms, thinly sliced (about 1½ cups)
  • 4 cups coarsely chopped Swiss chard or spinach
  • 1½ cups chopped fresh basil (about 40 medium-large leaves)
  • 1 ounce raw, unsalted cashews (about ¼ cup; for garnish)
  • For the sauce:
  • 2 cups unsweetened nondairy milk (I like soy milk)
  • 2 ounces raw, unsalted cashews (about ½ cup)
  • 1 tablespoon dried Italian herb seasoning
  • 1 teaspoon granulated onion
  • ½ teaspoon granulated garlic


  1. To make the sauce, place all of the sauce ingredients (nondairy milk, cashews, Italian herbs, and granulated onion and garlic) into a blender, and set aside (so the cashew can soften).
  2. To slice the potatoes thinly and uniformly (like potato chips), I suggest using a mandolin slicer. If you do not have a mandolin, use a chef’s knife and aim for about a 1/8-inch thickness. Place the sliced potatoes, onions, mushrooms, and chopped chard/spinach and basil each into their own bowls.
  3. Preheat the oven to 375°F. Set aside a 13×9-inch baking dish (you do not need to oil or use parchment paper).
  4. Blend the sauce thoroughly until smooth. Next, we will prepare the casserole by creating layers as follows in steps 5 through 9:
  5. Pour a thin layer of sauce into the dish so it coats the bottom evenly.
  6. Layer one-third of the potatoes across the bottom of the pan, only slightly overlapping them. Use the thickest and biggest potatoes on the bottom.
  7. Next, add half of the onions, half of the mushrooms, half of the basil, and half of the chard/spinach. Pour about half of the remaining sauce over this.
  8. Next, add another layer, starting with a layer of potatoes (use the smaller and any odd looking potatoes for this layer), followed by the remaining onions, mushrooms, basil, and chard/spinach.
  9. Now, finish the casserole with a final layer of potatoes, creating a nice-looking “roof” to your casserole. Pour the remaining sauce evenly over the top.
  10. Cover the dish with a glass lid (if it has one) or aluminum foil (make sure the foil doesn’t touch the food, and seal the edges). Bake for 30 minutes.
  11. Remove the foil (careful of the hot steam). Using a rotary cheese grater to create a cheese-like topping, grate the ¼ cup cashews evenly across the top, and cook uncovered for 5 minutes more, or until the potatoes are tender when pierced with a knife and the edges are lightly browned. Let stand for 5 minutes before serving.


For a lower-calorie dish, in the sauce use 1 cup water and 1 cup nondairy milk, and/or decrease the amount of cashews from ½ cup to ¼ cup. You can also omit or reduce the grated cashews on top.

If you have a high-sided (deep-dish) 9x13 casserole dish, this would be a great use for it. The layers can get pretty high, but if using a regular 9x13 dish (not deep dish), just press down on the layers with your hands before placing the last layer of potatoes.


  • Serving Size: 1/8 recipe
  • Calories: 155
  • Sugar: 2.9g
  • Sodium: 52mg
  • Fat: 6g
  • Saturated Fat: 1g
  • Carbohydrates: 21g
  • Fiber: 3.1g
  • Protein: 6.7g
  • Cholesterol: 0