When roasting sweet potatoes, leave the skins on for greater nutrition (and better texture). High heat muddies the fresh flavors of the herbs and lemon zest in pesto, so spoon it on just before serving.
Preheat oven to 425 degrees. On a rimmed baking sheet, toss sweet potatoes with 1 tablespoon oil. Season with salt and pepper. Roast, turning once, until golden and tender, 23 to 25 minutes. Let cool slightly.
Meanwhile, pulse parsley, garlic, walnuts, zest, and lemon juice in a food processor until coarsely chopped. Drizzle remaining 6 tablespoons oil and process until combined. Season with salt and pepper.
Top roasted sweet potatoes with 2 tablespoons pesto.
Brush this pesto on any roasted vegetable or piece of fish. You can also stir it into tuna salad.