In a large saucepan of boiling water, cook the potatoes until tender, about 12 minutes; drain well.: You'll hear a gentle bubbling as the water moves, and when the potatoes are tender a paring knife will slide through with little resistance. Boiling evenly cut 1 inch pieces ensures uniform doneness and prevents some pieces from overcooking while others are underdone. If the water is too vigorous the potatoes can break apart and become waterlogged, so aim for a steady boil. A common mistake is starting with very cold, rapid boil and not timing properly which leads to undercooked centers.
Return the potatoes to the pan and add the goat cheese, milk, and butter; mash until smooth. Add more milk if it's not quite creamy enough for you. Mix in chopped sage and salt; season to taste with additional salt and pepper, if desired.: As you mash, you should smell the tang of melting goat cheese and see the mixture turn creamy. Warm milk and hot potatoes help everything blend into a silky texture, while the butter melts and adds gloss. Mashing immediately back in the hot pan allows residual heat to help melt the cheese; if you stop too soon you might end up with lumps. Overworking can make potatoes gluey, so aim for smoothness without excessive pounding.
Mound the potatoes in a bowl. Garnish with fresh sage sprigs.: When you spoon a bit of mash onto a plate it should hold shape but still feel pillowy and moist. Adding warm milk a tablespoon at a time gives you control over texture, and the aroma becomes richer as fat integrates. A common pitfall is adding too much liquid at once which makes the mash thin and loses the satisfying body of a proper mashed potato.
Mix in chopped sage and salt; season to taste with additional salt and pepper, if desired.: The chopped sage should release its herbal notes as it warms against the mash, giving an aromatic lift. Stirring gently preserves little pockets of herb without smearing them into invisibility. Taste for seasoning because the goat cheese may already contribute salt, and you want a balanced finish. Over seasoning at this stage is easy, so add small amounts and re taste between additions.
Mound the potatoes in a bowl.: When you transfer the mash, it should glisten slightly from the butter and hold a soft mound shape, inviting spoonfuls. The visual cue of a smooth dome with small peaks means you achieved a creamy texture. If the surface looks dry, a small pat of butter or a splash of milk will revive it. A mistake I see is leaving the mash in the pot too long which causes cooling and a firmer, less inviting texture.
Garnish with fresh sage sprigs.: Placing whole or partial sage sprigs on top adds visual charm and a burst of aroma as guests dig in. The contrast of green against creamy white makes the dish feel seasonal and thoughtful. Avoid heavy garnishes that fall into the mash and wilt; light sprigs look and taste best.