Preheat your oven to 425 degrees Fahrenheit. This temperature matters — lower and the sweet potatoes will steam and stay soft. You need that high heat to get the edges crispy and the bottoms caramelized.
Arrange the sweet potato rounds in a single layer on a large parchment-lined baking sheet. This is the most important step, and it's where people usually go wrong: do not overlap them. Every round needs direct contact with the hot sheet pan. If they're piled on top of each other, they'll steam instead of roast and you'll end up with soft, floppy sweet potatoes instead of crispy ones. If you need to use two sheet pans, use two sheet pans.
While the sweet potatoes roast, make the pesto. Add the walnuts to a food processor and pulse a few times until they're broken into small pieces but not powder. Then add the basil, parsley, garlic, lemon juice, nutritional yeast, and salt. Pulse a few more times while drizzling in the olive oil through the feed tube. You want a texture that's somewhere between chunky and smooth — rustic, with visible herb flecks and small walnut pieces. Not a puree. The textural contrast against the smooth sweet potato is part of what makes this work.
Transfer the roasted sweet potato rounds to a serving platter or board. Dollop a small spoonful of pesto onto each round — about a teaspoon per coin. You can also drizzle the pesto over the whole platter more loosely if you prefer a less precious presentation. Scatter a few extra walnut pieces over the top, add a few small basil leaves if you want it to look especially beautiful, and finish with a light squeeze of lemon juice.
