In a large bowl, mix the water and salt to dissolve it. Set aside. Peel the potatoes and grate them using a food processor, box grater, mouli, etc. Immediately add the grated potatoes to the salted water; set aside for about 5 minutes while you prep the onion and bell pepper (if using).

Lay out a clean towel (or paper towels). Scoop handfuls of potato out of the water and squeeze as dry as possible. Spread them out on the towel as you go to dry further.

In a 12-inch non-stick skillet, melt half the butter over medium heat and add the onion and bell pepper (if using). Saute for a couple of minutes, then add the drained potatoes in a single layer. It should be about 1/2 to 3/4 inch thick. Sprinkle with paprika, then cover the pan tightly and continue to cook for about 15 minutes.

Check it once or twice to ensure the potatoes aren’t sticking, and use a spatula to tuck in any stray bits of potato around the edges. Don’t stir or otherwise disturb the potatoes. Add a little more butter if needed to prevent sticking.

The bottom of the rösti should be golden brown at this point. Spray a dinner plate with cooking spray, and carefully slide the entire rösti onto it. Add the remaining butter and melt it, then carefully invert the potatoes back into the skillet (paprika side down).

Continue to cook over medium heat for another 10 minutes or so, uncovered, until the bottom is browned. Slide the rösti onto a serving plate, garnish with chopped parsley and serve.