Steam potatoes until half cooked. Grate potatoes on a coarse grater and season with salt and pepper, thyme, sage and egg yolk. Mix well.
Overfill a 10cm rosti pan as mixture will shrink down during cooking. Press down firmly to lock potato together.
Remove potato from pan and grease pan with a little oil and butter. Place over medium to high heat, add rosti and cook until golden brown. Remove rosti, add a little more butter and oil, turn rosti over, return to pan and cook for 5 minutes, swirling the pan to reshape the rosti.