1 pound sweet potatoes, peeled, halved lengthwise, and cut crosswise into ½-inch slices
1/3 cup walnuts, shelled
2 cloves garlic, minced
12 ounces fresh shiitake mushrooms, stems discarded and caps thickly sliced
1/4 teaspoon salt
12 cups spinach leaves
4 teaspoons olive oil
1/2 cup red wine vinegar
1 tablespoon dijon mustard
Preheat the oven to 400 degrees. Place the cut sweet potatoes on a baking sheet coated with nonstick cooking spray and bake until tender, about 15-20 minutes. Remove the potatoes from the oven and cool. Toast the walnuts in a separate pan in the oven until crisp, about 5-7 minutes. Coarsely chop the nuts when they are cool enough to handle.
Heat a large nonstick skillet, coated with cooking spray, over medium heat. Add the garlic and cook until fragrant, about 30 seconds. Add half the mushrooms, sprinkle them with the salt, and cook until they begin to soften, about 4 minutes. Add the remaining mushrooms and cook until all the mushrooms are tender, about 5 minutes.
Place the spinach in a large bowl. Add the sweet potatoes and walnuts. Remove the mushrooms from the skillet with a slotted spoon and add to the bowl with the spinach. In a separate small bowl, whisk the olive oil, vinegar, and mustard. Drizzle over the salad mixture and combine.