Heat 2 tablespoons of olive oil in a cast iron skillet. Sauté leeks for a few minutes, add cabbage and let cook for about 5 minutes. Add garlic, cook until fragrant (about 30 seconds) then add kale. Cook for another 5 minutes, until the kale is a bit wilted. Add salt and pepper (about 2 teaspoons of salt and 1/2 teaspoon of pepper)
Add mashed potatoes to the kale mixture and mix in plant-butter and more olive oil, adjusting seasoning to taste.
Place cast iron in the oven and bake for 25 minutes.