Place the potatoes in a large pot and cover with cold water by 1 inch. Bring to a boil and cook until the potatoes are fork tender, about 15 minutes. Fold in the spinach and cook for an additional minute.: You will notice the water line just above the diced potatoes , a simple setup that ensures even heating from the inside out. Starting cold avoids a mushy exterior and undercooked core, because sudden heat can cause the outside to overcook while the center remains firm. You should hear a gentle simmer as the pot comes to temperature, and the steam will carry a clean, starchy aroma. Watch for the water to come up to a steady simmer instead of a violent boil, which can jostle the pieces and cause breakage. A common mistake is cutting the potatoes into uneven pieces, so take a moment to dice them uniformly for consistent cook times.
Drain the potato mixture and return to the pot along with 1/4 cup stock, 1 teaspoon salt, and the oil. With a fork or masher, smash the potatoes until semi-smooth. Add more liquid if the potatoes seem too thick and dry. Fold in the butter and taste for seasoning.: As the pot moves from a simmer to a rolling boil, the sound becomes livelier, and the steam will smell faintly sweet and earthy. Fork tender means a fork glides in with little resistance and the potato piece breaks apart easily when prodded. This texture indicates the interior starches have hydrated and softened enough for mashing without turning gluey. If you overcook and the pieces fall to mush, the mash can become sticky rather than light, so set a timer and test early. Be mindful that altitude and potato size can shift the time slightly.
Fold in the spinach and cook for an additional minute: The moment you add the spinach , the pan brightens; the leaves wilt almost immediately and release a fresh, green aroma. Cooking just a minute keeps the color vivid and prevents bitterness that can develop from prolonged heat. You will see the leaves shrink dramatically, and the texture shifts from crisp to tender. A pitfall is leaving the spinach in too long, which can cause a dull color and a soggy mouthfeel, so watch closely and remove it as soon as it softens.
Drain the potato mixture and return to the pot along with 1/4 cup stock, 1 teaspoon salt, and the oil: When you drain, you will notice the steam is scented with both starch and green notes, and the pot will feel warm and ready. Returning everything to the pot lets you control the mash consistency from the heat retained in the vessel. Add the initial stock and olive oil so the mixture loosens and gains savory depth, and the sea salt starts integrating. If the pot feels too dry or the mash looks crumbly, gradually add more stock rather than pouring it all at once. Over-wetting here leads to a gummy texture, which is hard to correct later.
With a fork or masher, smash the potatoes until semi-smooth: As you press and mash, listen for a slight tearing sound and watch the lumps give way to creamy ridges, releasing steam and that comforting potato aroma. Semi-smooth means there are still small curds for texture, rather than a completely puréed silk. This balance keeps the mash interesting and avoids a gluey mouthfeel. If you use a blender or food processor, you risk overworking the starches which creates a gummy result, so stick with a fork or hand masher for best texture. If lumps persist, mash gently in short bursts instead of vigorous overmixing.
Add more liquid if the potatoes seem too thick and dry: The visual cue here is dryness, where the mash holds clumps and does not spread easily on a spoon. Adding additional stock in small spoonfuls will loosen the texture while keeping the flavor intact. Each splash will warm and integrate, making the mash silkier and softer in the mouth. A mistake to avoid is pouring too much at once, which can quickly turn the mash soupy and dilute its flavor. Go slowly, tasting as you go to reach your perfect consistency.
Fold in the butter and taste for seasoning: When you fold in unsalted butter , it melts into the warm mash, creating a glossy sheen and rounding the edges of the flavor. The mouthfeel becomes richer and slightly decadent, without adding heaviness if used judiciously. Taste now for final seasoning because the butter can mute or enhance salt perception; adjust with a little more sea salt if needed. Avoid over-salting at this stage, and remember you can always add more but you cannot remove it.