Preheat your oven to 350F (180C). Line the bottom of an 8" x 8" baking pan with parchment paper.
For the strawberry filling: Mix the strawberries, maple syrup, corn starch, and lemon juice in a medium saucepan until the cornstarch is dissolved. Put over medium heat and cook for 3 - 5 minutes, stirring frequently, until the liquid turns into a thin gel and the strawberries begin to break down. Remove from the heat, stir in the vanilla extract, and set aside.
For the oat mixture: Add the flour, oats, butter, sugar, baking powder, and salt to a large bowl. Use a pastry cutter or a fork to mash the butter into the mixture until you reach a crumbly texture and everything is mixed in.
To assemble: Pour half of the oat mixture into the prepared baking pan and pat down. Pour all of your strawberry sauce on top and spread around to evenly distribute. Sprinkle the remaining oat mixture on top of the strawberry layer, and lightly pat down. Bake for 35 to 40 minutes, until lightly browned on top. Let cool before serving and slicing into bars. Store the cooled strawberry bars in an air-tight container in the fridge for up to 5 days or they can be frozen.
Strawberries: Frozen strawberries tend to be more watery so if using frozen you might need to cook the strawberry mixture a little longer to reach the correct consistency. You can also substitute strawberries for any other kind of berry or sliced peaches.