Combine all the ingredients for the dough in a large mixing bowl or the bowl of a stand mixer. Mix and knead until a soft, smooth dough forms. Transfer the dough to a lightly greased bowl, cover and set aside to rise for 90 minutes or until puffy but not necessarily doubled in volume.
Heat oil in a pan, add the onion and cook till it turns translucent. Add the garlic and cook for a minute.
Next add the boiled and mashed potatoes and season with salt and pepper. Cook covered for 4~5 minutes for the flavors* to mingle.
Cool the filling to room temperature and stir in the cheese.
Divide the dough into 8 equal pieces; shape each piece into a ball and place them on a parchment lined baking sheet leaving about 1" inch between them. Cover and rest for 15 minutes.
Shape each ball into a flattened round about 5" in diameter, brush the surface with some milk and place 1~2tbsp filling onto the center. Pull the dough over the filling, pinching 2 opposite edges together tightly, to seal in the filling. It should look like a dumpling. Place the buns on the parchment lined baking sheet, cover and set aside for 1 hour or until puffy.
Preheat the oven to 400°F.
Brush the buns with milk (or egg wash) and bake them for 15~20 minutes or until golden brown. Remove from the oven and allow to cool for 5~10 minutes before serving.