Preheat the oven to 375 degrees. Place the potatoes in a large saucepan. Add enough cold water to cover by 1 inch; bring to a boil over high. Season with salt, reduce the heat to medium and simmer until tender, about 20 minutes.
Meanwhile, in a large bowl, mix the oats, flour, brown sugar, pepitas, chia seeds, rosemary, orange zest, 3/4 tsp. salt and 1/4 tsp. pepper. Add the butter; using your fingertips, mix the streusel until large clumps form.
Drain the potatoes. Press the potatoes through a potato ricer into a large bowl or transfer to a bowl and mash with a potato masher or electric mixer; season. Transfer to a 9-by-13-inch baking dish. Sprinkle the streusel on top. Bake until the topping is golden and crispy, 30 to 35 minutes.