In a large sauce pan, melt butter over medium-low heat to begin browning your butter. It will melt, then foam, then turn clear golden and finally start to turn brown and smell nutty. Watch the butter closely and stir every few minutes until browned, about 10 minutes. Set to the side to cool slightly while preparing the greens and potatoes.

Scrub any dirt off the potatoes before peeling and dicing them into ½ inch chunks. Bring a large pot of water to a boil.

Add the diced potatoes and cook until knife tender. This can take anywhere from 10 to 15 minutes.

While the potatoes cook, heat three tablespoons olive oil over medium heat in a large cast iron skillet. Once heated, add the sliced leeks. Cook, stirring occasionally, to allow the leeks to soften, about 3 minutes. Add the sliced cabbage and season with kosher salt and ground black pepper. Allow the cabbage to begin to caramelize, stirring occasionally, for 7 minutes. Add the spinach during the final minutes of the potatoes boiling to allow the spinach to wilt. Set the greens to the side.

Once the potatoes are tender, use a slotted spoon to remove the potatoes from the pot and place them in a large mixing bowl. Mash the potatoes using either a rice or a masher. If using, add in the grated Irish cheddar and mix to allow cheese to melt gently into the hot mashed potatoes.

Pour in the browned butter followed by the room temperature milk. Using a fork or silicone spatula, milk the butter and milk into the potatoes. Add the caramelized cabbage, spinach, and leeks and stir everything until well combined. Taste and adjust seasoning with a pinch more kosher salt and ground black pepper as desired.

Serve immediately in your desired serving bowl or platter. Garnish with freshly chopped chives.