Make the chive sour cream: In a medium bowl, combine the sour cream, chives, mint, and kosher salt. Mix thoroughly and set aside.
Roast the lemon: Preheat the oven to 400°F.
Place the lemon slices in a single layer on a baking sheet, drizzle with the olive oil, and season with the kosher salt. Roast until lightly golden, about 5 minutes. Set aside.
Make the rösti: Wash and dry the sunchokes, then grate them into a medium bowl (no need to peel them). Peel the potatoes and grate them and the onion into the same bowl as the sunchokes. Transfer the grated mixture to a cheesecloth or clean kitchen towel and squeeze out all of the liquid. Really squeeze hard! The more moisture you can get out now, the crispier the rösti will be.
In a medium bowl, mix together the squeezed-out sunchokes, potatoes, and onion with the eggs, flour, kosher salt, and pepper until thoroughly combined. In a medium nonstick skillet, heat 2 tablespoons of the grapeseed oil over medium-high heat. Reduce the heat to medium-low and add half of the sunchoke mixture and press it down into the pan so it’s in an even layer and really comes together. Fry until golden and crispy around the edges, 8 to 10 minutes. Carefully slide the rösti onto a large plate and flip the rösti back into the pan. Fry the second side until crisp, pressing down with a spatula as it cooks, another 5 minutes. When the rösti is done, slide it onto a plate and repeat with the remaining 2 tablespoons grapeseed oil and sunchoke mixture.
To serve: Cut the roasted lemon slices into quarters. Season the rösti with pepper and sea salt, then serve with the chive sour cream, fresh mint, and a sprinkle of the roasted lemon pieces.