Preheat oven to 425* F. Grease a large rimmed baking sheet.
Wash sweet potatoes, scrubbing skin well. Cut into 1-inch slices and place on the prepared baking sheet.
Pour a few tablespoons of water into the baking sheet, then place into the oven. Bake for 30 minutes, until sweet potatoes are soft.
Meanwhile, make the pesto: use a food processor to combine walnuts, 1/4 cup parmesan, olive oil, garlic cloves, lemon juice, and salt. Pulse until well-combined. Then add basil leaves and spinach and pulse again until a pesto is formed.
Remove sweet potato rounds from the oven, flip them over, and smash with a fork or potato masher.
Spread each smashed sweet potato with a spoonful of pesto, then sprinkle a little extra parmesan cheese on top, if desired.
Place the sweet potatoes back into the oven (still at 425*F) and bake for 5-10 minutes.
Alternate cooking method: instead of baking the sweet potatoes to initially cook them (as in step 3), you can boil them. Place them into a boiling pot of water and allow them to cook until softened, about 20 minutes. Then drain and dry and proceed with the regular directions.