You only need five ingredients for this super-simple crisp: blueberries (yes, you can use frozen!) for the filling, and butter, flour, sugar, and cinnamon for the crumb topping.
Preheat oven to 375 degrees. Butter a 9-inch round baking dish; set aside.
Combine the flour, sugar, and cinnamon in a bowl; blend in the butter with a pastry cutter or your fingers until the mixture is crumbly.
Place the blueberries in the prepared baking dish and sprinkle the crumb mixture over them. Bake 30 to 35 minutes. Serve with ice cream, if desired.