Heat oven to 400°F . Peel potatoes and grate on the large holes of a box grater. Soak in water to cover for 5 minutes. Lift out with your hands and squeeze out liquid. Then squeeze out excess liquid in a clean kitchen towel. Or paper towel

Add potato mixture, and cook, pressing down lightly with large spatula. Until bottom is golden and releases easily from pan, about 4 minutes.

Invert Rosti onto a plate . Add remaining 1 tablespoon butter and oil and swirl to coat pan.carefully slide Rosti back into pan uncooked side down and cook until starting to brown underneath , about 3 minutes.

Transfer to oven ( since you are using ovenproof pan) and cook until deep golden brown. About 12-15 minutes

Meanwhile , sauté mushrooms. Melt the butter in a medium skillet over medium high heat. Cook leek until translucent, stirring, about 1 min. Add mushrooms , season with salt and pepper, sauté until golden, about 4 mins. Stir in stock and cook until reduced and slightly thickened and mushrooms have softened, 3, 4 mints .

Slide baked Rosti out of pan onto a serving platter. Spoon dollops of goat cheese in center of Rosti. And spoon mushroom mixture over top. Cut into wedges and serve immediately .