Cut the gold potatoes into roughly 1-inch chunks—keeping them uniform ensures even cooking. Mince the garlic cloves finely so they cook evenly and distribute their flavor throughout the potatoes. Measure out your milk, cream, coriander, and have your mustard, butter, and olive oil ready. Starting with prep complete eliminates scrambling while your potatoes cook.

Add the potato chunks, minced garlic, and coriander to a pot, then pour in the milk and heavy cream until the potatoes are just covered. Bring to a gentle boil over medium-high heat, then reduce to medium and simmer for 15-20 minutes, stirring occasionally to prevent sticking on the bottom. The potatoes are done when they break apart easily with a fork and are completely tender—don't rush this step as undercooked potatoes won't mash smoothly.

Carefully drain the potatoes through a fine-mesh strainer, reserving the cooking liquid in a bowl—this liquid is flavored gold and will give your mash incredible creaminess and taste. Return the drained potatoes to the pot and mash vigorously with a potato masher until you reach your desired texture. I prefer leaving a few small lumps for a rustic feel, but go ahead and mash smooth if that's your preference.

While the potatoes are still warm, stir in the butter, olive oil, and stone ground mustard, folding gently to incorporate. Gradually add the reserved cooking liquid from Step 3, a few tablespoons at a time, stirring until you reach a smooth, creamy consistency—you may not need all of it. The combination of butter and olive oil creates a luxurious mouthfeel, while the mustard adds subtle depth without being obvious.

Taste your mash and season with kosher salt and freshly ground black pepper to your preference—start conservative and build up, as the reserved liquid already carries salt from the milk and cream. Transfer to a warm bowl and serve immediately while the potatoes are still steaming and creamy.