Heat oven to 425 degrees. Cut each sweet potato in half lengthwise, then in half again (you should have 4 segments), then slice each segment into 3 or 4 wedges, so they are about ½ inch thick. Place them on a sheet pan.
Cut the red onion in half and slice into ½-inch-thick pieces, similar in size to the sweet potato. Place on the sheet pan with the sweet potato.
Drizzle with olive oil and add the harissa paste, cumin, 1 teaspoon of salt and a few pinches of black pepper. Using a large spoon (or clean hands), toss everything together until the sweet potatoes and onions are well coated, arranging everything in a single layer.
Place in the oven and roast for 10 minutes. Remove the sheet pan from the oven, carefully toss and return to the oven for 15 to 18 minutes, until the sweet potato is tender and crispy around the edges.
To serve, squeeze over the juice from the lemon half. Taste and season with more salt and black pepper, if needed. Top with the feta, if using, a dollop of harissa and finish with a handful of cilantro or mint leaves.