How to Make It
Preheat oven to 375°F. Place sweet potatoes and water in a large saucepan; bring to a boil over high. Cover and reduce heat to medium-low; simmer until sweet potatoes are very tender, about 25 minutes. Drain. Return sweet potatoes to pan; mash using a masher to desired consistency.
Whisk together eggs, cream, 1/4 cup of the sugar, and 2 teaspoons of the salt in a medium bowl. Whisk in 2 cups of the mashed sweet potatoes 1/2 cup at a time. Add mixture to remaining mashed sweet potatoes in pan; stir until well combined. Add 1/2 cup of the butter, stirring until melted. Transfer mixture to a 13- x 9-inch baking dish coated with cooking spray.
Stir together oats, flour, cinnamon, and remaining 3/4 cup sugar and 1/2 teaspoon salt in a medium bowl. Add remaining 1 cup butter; work butter into oat mixture using your fingers until well combined. Sprinkle evenly over casserole in baking dish. (If making ahead of time, cover casserole, and store in refrigerator up to 4 days.)
Bake in preheated oven until top is browned, 50 to 55 minutes. Let stand at room temperature 10 minutes.