• Preheat the oven to 375° F (190° C).

Make the crumble

• Add softened butter, flour, oats, brown sugar, cinnamon, nutmeg, salt, and baking soda to a medium-sized mixing bowl. Mix with the back of a fork or a pastry cutter until crumbly.

Make the berry filling

• In a separate bowl, add 2 1/2 cups of berries of choice. (I use 1 1/4 cup blueberries and 1 1/4 cup raspberries) Sprinkle the berries with 2 tbsp of sugar, 1 tbsp of cornstarch, and 2 tsp of lemon juice, then toss to coat.

Put it together

• Line the bottom of an 8" x 8" square metal baking pan with parchment paper, then pour 1/2 of the crumble mixture into the pan, spread it evenly, and press it down firmly.
• Spread the berry mixture evenly over the pressed crumble mixture.
• Top the berry layer with the remaining crumb mixture.
• Bake uncovered for 25-28 minutes until the top is slightly brown.
• Allow it to cool completely before glazing it and cutting it into squares.

For the glaze (optional)

• Mix 1/3 cup powdered sugar with 2 teaspoons of lemon juice until smooth. Drizzle over the berry bars is desired.

To cut the bars

• Remove the parchment paper from the baking dish and lay it flat on the countertop. Using a large knife, cut it in half in both directions to make 4 large squares. Then cut those squares in half in both directions, making 16 equal squares.