When cool enough to handle, peel the potatoes, cut them into large chunks, and place in a bowl. Mash coarsely and stir in the rice milk.
Heat half of the oil in a large skillet or steep-sided stir-fry pan. If using leeks, add them first and sauté over medium heat, covered, for 2 to 3 minutes, or until wilted.
Otherwise, add the scallions to the skillet followed by the cabbage and kale. Add just a smidgen of water, cover, and cook until the cabbage and kale wilt down a bit, about 3 minutes.
Add the potatoes to the skillet, stir them in with the veggies, and turn the heat up to medium-high.
Cook without stirring until the bottom of the mixture gets nicely browned. Stir and allow more of the mixture to brown. If need be, give it another stir and let more of the veggies brown, until they are to your liking. Season with salt and pepper and serve at once.