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 and 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.
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 done.
When quinoa is finished cooking 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.
Remove from heat and add to the cooked quinoa. 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, spinach and white wine mixture to the sweet potatoes. Stir to combine. Add remaining reserved liquid or water to potato mixture, ½ cup at a time until the potatoes reach the desired pureed consistency. Taste and add salt and pepper as needed.
Transfer the mixture to a sprayed 8x8 baking dish or equivalent.
Sprinkle over the top of the dish and place in the oven to cook for 30 minutes or until gratin is bubbly and golden on top.
Tips for BEST Results:Don’t forget to reserve the cooking water from your sweet potatoes. Using this liquid to cook the quinoa and add back in later works to retain the nutrients lost from the sweet potatoes in the cooking water.Choose dark orange and dark skinned sweet potatoes (maybe called yams) when making this recipe. These sweet potatoes contain more moisture which means they mash much better and also are sweeter in flavor. Mash the sweet potatoes well for a creamier consistency. Add additional reserved liquid to the final mixture before transferring to a casserole dish. The consistency of the mashed potatoes should be loose and easy to stir. This keeps the final dish from being overly dry.