Step 1
In a small bowl, mix mascarpone, sour cream, horseradish, milk and lemon juice. Season with salt and pepper.
Step 2
Using large holes on a box grater, shred potatoes and shallots. Place potato mixture into centre of a kitchen towel. Gather corners of towel, and squeeze mixture in towel over the sink, extracting as much liquid as possible. Transfer mixture to a medium bowl. Drizzle 1½ tbs of butter into mixture and then mix in 1¼ tsp of salt.
Step 3
Heat a deep 25cm-diameter frying pan over medium heat and add 2 tbs of oil. When oil is hot, add half of potato mixture and, using a spatula, press down to evenly distribute mixture. After 30 secs, add remaining potato mixture and press down to form a cake. Cook for 8 mins or until potato mixture is crisp and deep golden brown on bottom. Place a large plate over pan and carefully invert rösti onto plate.
Step 4
Return pan to medium heat and add remaining butter. Slide rösti, browned side up, back into pan. Drizzle remaining 1 tbs of oil around edges of rösti. Cook for 8 mins or until rösti is crisp and deep golden brown on bottom and potato is tender in the centre.
Step 5
Slide rösti onto a stack of paper towels to drain. Place on a cutting board. Slice and serve with horseradish mascarpone cream. Sprinkle with chives.