Sweet Onion Casserole

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Why You’ll Love This Recipe

Onion-forward flavor: Using 8–12 onions makes this dish naturally sweet, caramelized, and deeply satisfying.
No canned soup: The creamy base is made from scratch with mushrooms, garlic, herbs, and rustic cashew milk.
Better bread: Toasted whole-grain sourdough adds fiber, flavor, and structure instead of refined white bread.
Nourishing comfort: Rich in phytonutrients, prebiotic fiber, healthy fats, and antioxidants — this casserole comforts without the processed burden.
Flexible: Add a sprinkle of raw-milk cheddar for richness, or keep it fully plant-based with nutritional yeast.

Author: Shaunie

Ingredients

Scale

For the Rustic Cashew Milk:

1 cup raw cashews, soaked overnight (or at least 4 hours) in water
2 cups fresh water

For the Mushroom Soup Base:

2 tbsp olive oil or ghee
2 cups cremini or button mushrooms, finely chopped
2 cloves garlic, minced
Pinch of sea salt
3 tbsp whole-grain flour or chickpea flour
½ tsp thyme
½ tsp rosemary
¼–½ tsp sea salt (to taste)
¼ tsp black pepper
Optional: 2–3 tbsp nutritional yeast for a cheesy note

For the Bake:

8–12 large Walla Walla onions (or other sweet onions), sliced
1–2 tbsp olive oil or avocado oil (for sautéing)
4 cups whole-grain sourdough bread, cubed & lightly toasted
Optional topping: ½ cup raw-milk cheddar or plant-based parmesan sprinkle

Instructions

Step 1: Soak the Cashews

Place 1 cup raw cashews in water and soak overnight (or at least 4 hours). Drain before use.

Step 2: Make Rustic Cashew Milk

Blend soaked cashews with 2 cups fresh water until smooth and creamy. Set aside.

Step 3: Caramelize the Onions

Slice 8–12 onions. Heat 1–2 tbsp olive oil in a large skillet over medium heat. Add onions and a pinch of salt. Cook slowly for 20–25 minutes, stirring occasionally, until golden and soft.

Step 4: Cook the Mushrooms & Garlic

In a medium pot, heat 2 tbsp olive oil or ghee. Add 2 cups chopped mushrooms, 2 cloves garlic, and a pinch of salt. Cook 6–8 minutes, until browned.

Step 5: Make the Mushroom Gravy

Sprinkle 3 tbsp flour over the mushrooms, stirring 1 minute. Slowly whisk in the cashew milk. Add ½ tsp thyme, ½ tsp rosemary, ¼–½ tsp salt, and ¼ tsp pepper. Simmer 5–7 minutes until thick and creamy. Stir in 2–3 tbsp nutritional yeast (optional for cheesiness).

Step 6: Combine

Fold caramelized onions into the mushroom gravy. Toss with 4 cups cubed, toasted sourdough bread.

Step 7: Bake

Transfer mixture to a greased 9×13” dish. Sprinkle with raw-milk cheddar or plant-based parmesan if desired. Bake at 350°F for 30–40 minutes, until bubbling and golden.

Step 8: Rest & Serve

Let sit for 5 minutes before serving. Pair with a green salad or roasted vegetables for a complete meal.

Tips for Success

1. - Take your time with the onions — slow caramelization brings out their natural sweetness.
  - Soak cashews fully for the creamiest milk. If in a rush, a 30-minute hot water soak works in a pinch.
  - Toast the bread lightly before mixing, so it holds up and doesn’t turn mushy.
  - Make ahead: Prepare up to the baking step, refrigerate overnight, then bake before serving

Sweet Onion Casserole