Preheat the oven to 400°F.
Cut the sweet potato into large wedges, toss with a couple glugs of neutral oil and a couple pinches of salt, and roast until caramelized and tender, 25 to 35 minutes, depending on the thickness of the potato.
In a medium frying pan, heat the neutral oil over medium-high heat until shimmering. Add the buckwheat and fry until golden brown and crunchy. Remove from the heat and add a few pinches of salt.
To serve, place the sweet potato wedges on a serving platter or plates, spoon the harissa over the top liberally (depending on your spice preference), and scatter the crisped buckwheat all over. Top with a handful of roughly chopped herbs, if desired. Serve warm or at room temperature.