Preheat oven to 375°F. Place 1 tablespoon of the melted butter in a 10-inch cast iron skillet or an 11 x 7-inch dish. Swirl it around or use a pastry brush to coat the skillet or dish with the butter.
In a large bowl, whisk together the oats, ¼ cup of the slivered almonds, cinnamon, nutmeg, baking powder, and salt.
In a medium bowl or large measuring cup, whisk together milk, honey, egg, 2 tablespoons of the melted butter, and the vanilla extract.
Stir the wet ingredients into the dry ingredients.
Arrange the sliced strawberries in the bottom of the buttered skillet.
Pour the oat mixture into the skillet and spread in an even layer.
Sprinkle blackberries, blueberries, raspberries, and remaining ¼ cup of almonds over top. Drizzle with remaining 1 tablespoon of melted butter.
Bake until bubbly and set, about 35 minutes. Let stand for at least 5 minutes. Serve with optional toppings.