Steps
1. Preheat the oven to 375°F (190°C).
2. Peel and dice the potato into small cubes, then boil in salted water until tender (about 10-12 minutes).
3. Drain the potatoes and mash them in a bowl with the butter, milk, salt, and pepper until smooth.
4. In a separate bowl, whisk the egg and mayonnaise together until well combined.
5. Fold the egg mixture into the mashed potatoes until evenly combined.
6. Transfer the mixture to a small baking dish and smooth the top.
7. Bake in the preheated oven for 20-25 minutes, or until the top is golden and slightly puffed.
8. Remove from oven, let cool slightly, and garnish with chopped herbs if desired before serving.