Prep the vegetables: Peel potatoes, carrots, and parsnips. Cut into even chunks about 1 to 1.5 inches so they cook at the same rate.
Boil until tender: Add all the vegetables to a large pot, cover with cold, generously salted water, and bring to a boil. Simmer 15–20 minutes, until a fork slides in easily.
Warm the dairy: In a small saucepan, gently heat the milk or cream with butter.
If using garlic, add it now to soften and mellow. Keep warm.
Drain well: Drain the vegetables thoroughly and return them to the hot pot. Let steam off for 1–2 minutes to remove excess moisture.
Mash to your style: Use a potato masher for rustic texture or a ricer for ultra-smooth results.
Start mashing before adding liquid.
Add butter and milk: Pour in about half the warm milk-butter mixture. Mash and stir. Add more as needed to reach your ideal creaminess.
Season: Stir in salt and pepper to taste.
Add herbs and lemon zest if using. Finish with a drizzle of olive oil for a glossy finish.
Taste and adjust: Add an extra pat of butter, a splash more milk, or a pinch of salt to balance sweetness and richness.
Serve hot: Transfer to a warm bowl and top with more herbs and pepper. Serve right away.