Prep: Preheat oven to 375℉. Line a 9 x 13 baking pan with parchment, ensuring their flaps are folded over the sides for easy removal. Spray bottom of pan with olive oil or your favorite non-stick cooking spray.
Dry Ingredients: In a large bowl, whisk together 2 ¾ cups all-purpose flour, ½ cup brown sugar, ½ cup organic cane sugar, ¼ cup rolled oats, 1 tablespoon ground flax and pinch kosher salt. Set aside.
Wet Ingredients: In a small bowl, whisk together 1 egg and 1 teaspoon vanilla extract. Pour into flour mixture and stir to combine. Add 1 cup salted butter (softened & cubed) to flour mixture and cut into flour with a fork.
Make Crust: Pour a little OVER half of the flour mixture into the prepared baking pan. Spread out evenly and press down firmly with your hands/fingertips to create the crust. Set aside the rest of the flour mixture.
Filling: In a medium bowl, combine 3 cups strawberries, 2 ½ cups rhubarb, ⅓ cup organic cane sugar, 1 tablespoon cornstarch, 1 tablespoon lemon juice, 1 tablespoon honey, 1 teaspoon orange zest, 1 teaspoon vanilla extract, and ¼ teaspoon ground nutmeg. Stir to combine. Pour over the top of the crust and spread out evenly.
Topping: Take the remaining flour mixture and add 3 tablespoons brown sugar. Use a fork to combine. Crumble the mixture evenly over the top of the fruit filling.
Bake: Place in the oven, middle rack, 38-40 minutes or until the top starts to become golden brown.
Serve: Remove and let cool completely before cutting into squares.
Notes
Brown Sugar: you can use coconut sugar in place of brown sugar, if you prefer, it will be a bit less sweet.
Gluten-Free: to make this gluten-free use 1-to-1 gluten-free flour and gluten-free rolled oats.
Dairy-Free: to make this dairy-free use a plant-based butter.
Storage: Store covered in the refrigerator for up to 5 days.