Spoon the blueberry mixture into an 8×11-inch baking dish. Set aside.
To make the topping, combine the flour, oats, and brown sugar, and stir to combine.
1 ½ cups old-fashioned oats, 1 cup all-purpose flour, 1 cup light brown sugar
Add the butter, and cut in until the mixture looks crumbly.
½ cup cold salted butter
Crumble the topping over the filling.
Bake for about 45-50 minutes, or until the filling is hot and bubbly all the way through and the topping looks crispy and light golden brown. If the topping begins to brown too quickly, tent with aluminum foil.
Notes
Blueberries: If using fresh blueberries, you'll want nice, ripe blueberries. If your blueberries aren't very plump, you can add a tablespoon or so of lemon juice to the filling. If using frozen blueberries, don't thaw before use. Also, be sure to shake off an excess ice so you don't end up with watery filling.
Lemon zest: The citrus helps to add a bit of brightness to the blueberry filling. You could use orange zest if you prefer.
Oats: I've only used old-fashioned oats to make the topping.