Place potatoes and garlic in a large pot and cover with water.: The smell as the water heats begins to coax out the garlic aroma even before the boil, giving a subtle backbone to the dish. Use cold water so the potatoes heat evenly from the inside out, preventing tough edges with raw centers. A common mistake is starting with hot water which can cause uneven cooking. Make sure all pieces sit in a single layer as much as the pot allows so they reach similar tenderness.
Bring to a boil. Reduce heat, cover, and simmer for about 20 minutes (or until potatoes are tender).: You will hear a steady simmer, and the surface will move with gentle rolling bubbles. The potatoes are ready when a fork slides into the largest piece easily, and you may notice a faint starchy scent. Overcooking can cause them to fall apart and become waterlogged, so check at the 15 to 18 minute mark to avoid mushy texture. Keep the lid slightly ajar once boiling reduces to avoid violent boil overs.
Drain potatoes and place them back in a pot.: When you drain, the room will carry a warm, earthy potato perfume. Returning them to the still warm pot allows residual heat to evaporate excess moisture, which helps achieve a fuller, less watery mash. A pitfall is leaving them in a colander too long where they can cool and stiffen; work promptly to the next step for best integration of dairy and fat.
Add butter, half and half, garlic salt, and pepper. Use a potato masher to mash potatoes until desired consistency and combine ingredients until they are the preferred consistency.: When you add the butter to the hot potatoes, you will hear a soft melting hiss and see glossy streaks as it pools. Pour the warmed half and half gradually so the potatoes absorb it without becoming gluey. The seasoning will start to bloom on contact, so taste as you go and adjust. Too much liquid too quickly is a frequent error, so add in stages and stop when the texture feels right under your spoon.
Stir in cheese and top with fresh parsley. ENJOY!: As you work the masher, listen for a soft tearing sound and observe the potatoes breaking into velvety pieces. Mashing releases steam and develops the creamy body, but overworking can make them gummy due to excessive starch release. If you want lumps, mash gently and stop early, if silky, press and smooth longer, but not endlessly. Avoid electric beaters here unless you want a gluey texture.
Stir in cheese and top with fresh parsley ENJOY!: The shredded Parmesan cheese melds in, releasing a nutty, salty aroma that rounds the flavors. When stirred off heat, the cheese melts into pockets, adding depth without separating oils. Finish with a sprinkle of fresh parsley to introduce a faint herbal brightness. A common oversight is adding cheese to an overly hot pot which can cause it to clump rather than integrate, so remove from direct heat first.