Preheat oven to 375 degrees. Add parchment paper to a 9 x 13 baking pan so enough overlaps the sides to use as a handle later.
Stir together oats, flour, almond meal, ½ cup granulated sugar, brown sugar, baking powder and cinnamon in a bowl. Stir in melted butter. Reserve ¾ cup of mixture in a small bowl. Press remaining mixture into bottom of parchment-lined baking pan.
In another bowl, combine berries, lemon zest and juice, ⅓ cup granulated sugar and cornstarch. Stir until well combined and then pour into baking pan. Distribute berries evenly in pan.
Add sliced almonds to reserved oat mixture and stir to combine. Evenly sprinkle mixture on top of blueberries. Bake for 45-50 minutes or until crumble topping is nicely browned. Cool completely. Store in refrigerator until ready to serve.
To make the glaze:
Whisk together powdered sugar, milk and extract in a small bowl. Drizzle over cut blueberry bars.
Notes
I like to eat these chilled from the fridge, but they are just as tasty at room temp.