Shred 450 grams of unpeeled Yukon gold potatoes using a food processor or box grater.
Squeeze all the water out of the shredded potatoes.
In a bowl, toss the potatoes with 1/8 cup chopped parsley, 1/8 cup chopped green onion, 1 teaspoon chopped rosemary, 1/2 teaspoon pepper, and 1/4 teaspoon salt until well combined.
Divide the potato mixture into four equal portions.
Heat half the butter in a large nonstick skillet set over high heat.
When the butter foams, add the potato mixture and use a silicone spatula to form four individual disks of equal thickness.
Reduce the heat to medium-high and cook for 4 to 5 minutes, or until the underside is set and a golden crust forms.
Add the remaining butter and gently turn the rosti.
Cook for an additional 5 minutes, or until the rosti are golden brown and bubbles actively form in the center of each disk.
Drain the rosti on paper towels.
In a separate bowl, combine 1/2 cup sour cream, 1/8 cup remaining chopped parsley, 1/8 cup remaining chopped green onion, and 1 1/2 teaspoons Dijon mustard.
Serve the herbed potato rosti with the sour cream mixture.