Turn the oven on to 180° C / 355°F.
Grate the potato on the grater (use the larger holes).
Squeeze the excess water from the shredded potato.
Finely grate half an onion.
Mix the grated potatoes, onion, sea salt, and pepper.
Heat a small frying pan (medium-high heat).
Add 25 grams of butter.
Once the butter is bubbly, top with the shredded potato mixture and fry until it becomes dry and wants to stick to the pan (about 5 minutes).
Place the muffin pan in the oven briefly.
Take out the pan and top each tin hole with a small piece of butter.
Use a teaspoon to ensure the melted butter covers the entire bottom of the muffin tin hole.
Top with a spoonful of the potato mixture and flatten with a spoon.
Sprinkle sea salt on each rosti.
Bake in the oven for 20-22 minutes until golden and bubbly.
Allow to cool slightly in the pan.
Lift the rosti carefully with a fork and place it on a piece of kitchen towel to soak any excess butter.