Preheat oven to 375°. In a 9- to 10-inch cast-iron skillet, swirl 1 tablespoon melted butter until coated.
In a large bowl, whisk together oats, ¼ cup almonds, pie spice, baking powder, and salt. In medium bowl, whisk together milk, honey, egg. 2 tablespoons melted butter, and vanilla. Stir milk mixture into oat mixture.
Add 1¼ cups blackberries to prepared skillet. Pour oats mixture all over. Top with remaining ½ cup blackberries and remaining ¼ cup almonds. Drizzle with remaining 1 tablespoon melted butter.
Bake until bubbly and set, 30 to 35 minutes. Let stand for 5 minutes. Serve with additional honey.
Recipe by Southern Cast Iron at https://southerncastiron.com/baked-blackberry-almond-oatmeal/