• Preheat the oven to 400 degrees.

Boil the Sweet Potatoes

• Wash and peel sweet potatoes. Cut into medium sized cubes and place into a pot. Cover with just enough water to cover the potatoes. Add a bay leaf, white miso and two cloves of garlic to the water. Heat to boil over medium high heat, then reduce heat when water boils to medium. Cook until potatoes are tender when pierced with a fork.
• When potatoes are ready, drain off the water, reserving it for later. Discard the bay leaf and garlic. Transfer the potatoes to a bowl and use a fork to mash the potatoes.

Make Quinoa and Breadcrumbs

• Rinse quinoa if needed and drain well.
• Place quinoa and 1 cup of reserved cooking liquid in a pan and bring to a boil. When it reaches a boil, reduce the heat to a simmer, cover and allow to cook for 10-15 minutes or until water is absorbed and quinoa is tender. Remove from heat and set aside.
• While the quinoa cooks, mix together breadcrumbs, finely chopped walnuts, parsley, thyme and garlic powder. Add salt to taste.

Prepare the Casserole

• Heat olive oil in a pan over medium-high heat. When oil is hot, add onion and garlic and cook for 3-5 minutes, until onions are translucent and slightly golden. Stir frequently to keep the garlic from burning.
• Mix together the cooked quinoa, onions and garlic, and chopped spinach.
• Combine white wine, soy sauce and nutritional yeast in a small bowl and mix to dissolve yeast.
• Add the quinoa mixture and white wine mixture to the sweet potatoes. Stir to combine. Add remaining reserved liquid or water to potato mixture one-half cup at a time until the potatoes reach the desired pureed consistency. Taste and add salt and pepper as needed.
• Transfer the potato mixture to a sprayed 8x8 baking dish or equivalent.
• Sprinkle the breadcrumb mixture over the top of the dish. Place in the oven to cook for 30 minutes or until gratin is bubbly and golden on top.