• Preheat the oven to 425 degrees F. Nestle a wire rack on top of a sheet pan and set aside.
• Place the peeled and diced potatoes in a large pot and cover with cold water. Heavily salt the water and bring to a boil over high heat. Boil the potatoes until fork tender, about 10 minutes. Drain the potatoes. Once drained, place a plate on top of the colander and firmly hold it down, then shake the colander as hard as you can without losing any of the potatoes until they are a bit messy. Transfer the potatoes to the prepared sheet pan and let dry for 20 minutes. Meanwhile, pour the oil onto a separate sheet pan and heat in the oven for 4-5 minutes.
• VERY carefully, remove the sheet pan from the oven with the hot oil and transfer the potatoes on top of the oil. Do this very gently so the potatoes/oil don't splatter. Gently mash the potatoes with the back of a spoon or flat surface until they fill most of the sheet pan, then season with kosher salt and tons of freshly cracked black pepper. Scatter the butter over top of the potatoes. Transfer back into the oven.
• Bake until the potatoes are crispy on the bottom, about 45 minutes. Give the potatoes a toss and continue to bake until they are crispy and golden almost all over, another 15-30 minutes. 5 minutes before they are done, add the cheese and place it back into the oven to melt.
• Let cool slightly, about 10 minutes. Season with more salt and pepper, add more cheese if desired (duh) and sprinkle with the lemon juice and zest. Serve with the green sauce.

For the Green Sauce

• Puree everything together in a blender and season to taste. Set aside until ready to use. Can be stored for 1 week in the fridge.