Heat the Oven - Set the oven rack to the center position. Preheat to 375ºF (191ºC). Lightly grease an 8-inch baking dish with baking spray, vegetable oil, or softened butter. Set aside.
Make the Filling - In a large bowl, combine blueberries, sugar, flour, lemon juice, lemon zest, cinnamon, and salt. Evenly spread the blueberry filling into the greased baking dish.
Make the Crisp Topping - In a medium bowl, mix together flour, rolled oats, brown sugar, cinnamon, ginger, salt, and nutmeg. Cut cold butter into small ¼-inch pieces and add to the flour mixture. Use your fingers to press the butter into the dry ingredients to form clusters, ranging from pea to larger kidney bean-shaped pieces. If needed, chill the topping for 10 minutes to firm up the butter, making it easier to form clusters.
Bake the Crisp - Evenly sprinkle the crisp topping over the blueberries. Bake until golden brown, and the berries are bubbling, about 35 to 40 minutes. If needed, loosely cover the top with foil if it's getting too brown.
To Serve - Wait 10 to 15 minutes before serving. Serve warm with desired toppings, such as whipped cream or ice cream.
Notes
Storing and Reheating: Cool to room temperature and refrigerate for up to 3 days. Reheat small portions in the microwave on high power until warm, 45 to 60 seconds.
Freezing: The unbaked blueberry crisp can be covered and frozen for up to 1 month. Transfer to the refrigerator 1 day before baking. A baked crisp can be cooled down and then frozen for up to 1 month. Reheat at 350ºF (177ºC) until hot.
Making in Advance: All components (filling and topping) can be assembled 3 days before baking. Refrigerate them separately or all in the baking pan.
Sweetener Options: Honey or pure maple syrup can be substituted for granulated sugar in the filling.
Make it Gluten-Free: Swap flour with 1:1 gluten-free baking flour or cassava flour for filling and topping. For the topping, you can use almond flour instead of all-purpose flour. For the filling, substitute 2 tablespoons of cornstarch for ¼ cup of flour.