For the oatmeal:
Butter, for coating the baking dish
1 1/2 cups old-fashioned rolled oats
1/2 teaspoon baking powder
1/2 teaspoon kosher salt
5 large egg whites, at room temperature
1/3 cup speculoos cookie butter
1 1/2 cup whole milk
1/2 cup heavy cream
1 teaspoon vanilla extract
1 1/2-inch fresh ginger, peeled and finely grated
Finely grated zest of 1 lemon
1 cup raspberries, plus more for garnish
For the crumble:
1/2 cup old-fashioned rolled oats
1/4 cup all-purpose flour
1/4 teaspoon ground ginger
1/4 teaspoon ground cinnamon
1/4 teaspoon kosher salt
4 tablespoons (1/2 stick) cold butter, cut into small cubes
1/2 cup pistachios, coarsely chopped
1/4 cup maple syrup
Make the oatmeal: Arrange a rack in the middle of the and heat to 350°F. Coat a broiler-safe 9 by 9-inch baking dish with butter; set aside.
Combine the oats, baking powder, and salt in a medium bowl; set aside. Beat the egg whites in a large bowl with an electric mixer until firm peaks form; set aside.
Whisk the cookie butter in a large bowl to soften and separate. Add the milk, heavy cream, vanilla, ginger, and lemon zest and whisk to combine. Mix in the oat mixture until thoroughly combined. Gently fold in the egg whites and 1 cup of raspberries.
Transfer to the prepared dish and gently smooth over with a spatula. Bake until the center is just set and the top is lightly browned, 10 to 15 minutes. Meanwhile, make the crumble.
Make the crumble: Whisk the oatmeal, flour, ginger, cinnamon, and salt together in a medium bowl. Add the cubed butter and work it in with your fingers until it resembles coarse crumbs. Add the pistachios and stir in the maple syrup.
When the oatmeal is ready, remove it from the oven. Sprinkle the crumble over the baked oatmeal, then bake for 5 to 8 minutes more. Turn the oven onto broil and broil until the crumble is deep golden-brown, about 5 minutes. Top with additional fresh raspberries before serving.