• Preheat the oven to 375°.

FOR THE CRISP TOPPING:

• In a medium bowl, whisk together ¾ cup all purpose flour, ⅓ cup sugar, ¼ teaspoon salt, and 1 teaspoon lemon zest. Add 4 tablespoons unsalted butter and toss it into the dry ingredients. Use your clean hands to rub and crumble the butter into the dry ingredients until it's coarsely combined and sandy looking.
• Add the ⅓ cup rolled or quick cooking oats and ¼ cup sliced almonds; toss to combine.

FOR THE BLUEBERRY FILLING:

• Rinse 5 cups blueberries under running water and drain; removing any damaged fruit. Skip this step if using frozen berries. Add 1 teaspoon lemon zest and ~2 tablespoons lemon juice; toss well to combine.
• In a separate bowl whisk together ¼ cup sugar and 1½ tablespoons cornstarch. Sprinkle over the berries and fold the sugar mixture into the fruit until it's evenly coated. (Note: if using frozen berries, add an extra 1½ teaspoons of cornstarch to the sugar mixture to compensate for extra moisture from the fruit).
• Transfer the berries to a 3-4 cup shallow oven-proof casserole dish. Spread into an even layer and top with the crisp mixture. Bake for 30-35 minutes or until the fruit is bubbling and the crisp topping is golden brown.
• Let the crisp rest a few minutes before serving, as the filling is super-hot. Serve as is, or with a spoonful of whipped cream or a scoop of vanilla ice cream.