Place top oven rack in the center position and pre-heat oven to 350°F.
Place blueberries in a large bowl. Add maple syrup, cinnamon, allspice, rosemary, and salt. Toss to combine and set aside.
Combine the topping ingredients in a separate bowl and stir until oats are thoroughly coated and all ingredients are well combined. Set aside.
Pour blueberry mixture into a 10” cast iron skillet and spread into an even layer. Spread the topping evenly across the top in a uniform layer, as well.
Place into pre-heated oven and bake for 30 minutes. If the top layer is nicely browned at this point, cover dish loosely with aluminum foil to prevent over-browning. Bake for another 15-20 minutes or until the blueberry crisp is nice and bubbling. Remove from oven and set aside to cool for 5-10 minutes.
Serve warm with some Greek yogurt, if desired. Enjoy!