If using leftover mash, use as is. If cooking a fresh batch of potatoes: cook in salted boiling water until tender. Roughly mash with the skins on.
Cube the butter into small pieces. You’ll be using a few cubes along the way as the bubble and squeak cooks. Add a few small cubes to a pan and sauté the onions. Once browned, add the kale and wilt for 1-2 mins. Retain the pan.
Mix the kale-onion mixture and the mashed potatoes in a bowl. Season with salt and pepper. Use your hand to bring it all together.
Add a bit more butter to the same pan and pile on the kale and potato mash. Flatten the top with a spatula. Cook over medium heat until the bottom browns (use a spatula to lift off a small piece to test the bottom).
Pinch some of the butter with your fingers and dot them around the edges of the pan. Also spread some over the top.
If cooking in the oven: (make sure your pan is oven-proof). Transfer the pan to the top rack of your oven and cook for 15-20 mins. It won’t be completely browned on top but it’ll start to crisp around the edges.
If cooking on the stove top: run a knife along the edges to loosen the sides. Shake the pan a few times to loosen the bottom. Hold a plate over the pan and quickly flip. If it breaks just add the pieces back and patch it up. Add a bit more butter and gently ease the bubble and squeak back into the pan to finish cooking the other side. Cook until the bottom browns.
Best served hot with runny eggs. Also reheats very well.
Notes
Chinese cabbage is a good substitute for kale in this recipe.