Boil potatoes: Place sweet potatoes and Yukon gold potatoes in two separate saucepans. Cover each pot of potatoes by 1-inch of water. Salt them generously. Bring them to a boil and let them cook until they are fork tender. Drain. Return potatoes to respective pans.
Mash potatoes: Preheat the oven to 425 °F. Whisk together the milk, heavy cream, and butter in a mixing bowl. Divide the mixture in between the saucepans. Using a potato masher or hand blender, mash potatoes until smooth. Season with salt and pepper.
Lightly spray a casserole dish with cooking spray. I used an oval-shaped (or 9 x 11 inch rectangular) casserole dish.
Layer potatoes: Spread mashed sweet potatoes in the prepared casserole dish. Using the back of a spoon or a spatula, smooth out the top into one even layer. Top with the mashed Yukon gold potatoes and spread it evenly to the edges, creating delicate divots and crevices.
Bake: Bake until golden, 20-25 minutes.
Fry leeks: Meanwhile, mix together the flour, leeks and a pinch of salt. Toss to coat. Heat the avocado oil in a medium saucepan over medium-high heat for 2 minutes. Add leeks into the oil and fry, turning occasionally, until crisp and golden around the edges. Transfer the crispy leeks onto a plate lined with a paper towel.
Finish and serve: Season the crispy leeks with additional salt and pepper. Sprinkle them over the potato casserole. Serve immediately.