Preheat oven to 350F and line a metal 8x8 inch baking pan with parchment paper.

Measure out flour, oats, baking powder, spices, and brown sugar into a large bowl and stir to combine.

Add in melted butter, maple syrup, and egg, then stir the mixture until fully combined.

Take one and a half cups of the mixture and press it evenly into the bottom of the pan to form the crust.

Reserve the remaining half cup of the oatmeal mixture to use as the crumble topping later.

Add the diced strawberries and rhubarb to a separate mixing bowl along with lemon juice and stir to coat the fruit.

Stir together granulated sugar and cornstarch in a small bowl, then pour it over the fruit and stir.

Pour the fruit filling evenly over the oatmeal crust and sprinkle the reserved crumb topping over the fruit layer.

Bake for 20 to 25 minutes until the top is golden, then let cool completely before slicing.