Wash and dry all produce. Halve, peel, and thinly slice onion. Trim, then thinly slice scallions, keeping greens and whites separate. Peel potatoes, then grate on the large holes of a box grater.

Heat a drizzle of oil in a large pan over medium-high heat (TIP: Use a nonstick pan if you have one.) Add onion and scallion whites, then season with salt and pepper. Cook, tossing, until translucent, 3-5 minutes. Meanwhile, place potatoes on a clean kitchen towel or large piece of paper towel and squeeze to wring out moisture.

Place potatoes and onion in a large bowl. Season with salt and pepper. Toss to combine. Wipe out same pan and heat another drizzle of oil and 1 TBSP butter in it over medium-high heat. Add potatoand-onion mixture to pan, pressing down with a spatula to create one large pancake. Cook until crisp and browned on bottom, 7-10 minutes.

Slide rösti out of pan and onto a plate. Add another drizzle of oil and another 1 TBSP butter to same pan over medium-high heat. Flip rösti into pan crispy-side up. Cook until crisp and browned on other side, another 7-10 minutes. Meanwhile, cut tomato into 8 wedges. Halve lemon, cutting one half into wedges

While rösti cooks, whisk together a drizzle of oil and a squeeze of lemon in a medium bowl. Season with salt and pepper. Add arugula to bowl and toss to coat in dressing.

Slide rösti onto a cutting board and cut into slices. Divide between plates. Top with prosciutto, tomato wedges, and scallion greens. Dollop with sour cream. Serve with salad and lemon wedges on the side.