Preheat oven to 375°F. On your cutting board, slice the potatoes thinly and fill the baking dish with half of the potatoes. Season with salt and black pepper.
Add all the broccoli and cauliflower evenly over the potatoes. Sprinkle half of your cheddar and mozzarella cheese on top.
Add the remaining potatoes, season again, and pour the milk or heavy cream evenly over the dish. Top with the remaining cheese.
Cover with foil and bake for 35-40 minutes. Remove the foil after 40 minutes and bake for another 10-15 minutes until the potatoes are tender, and the top is golden and bubbly.
Let sit for 5-10 minutes and serve!