Directions

• Preheat oven to 425° (F) and adjust the oven rack to the middle position. Place a sheet pan on the middle rack. Prepare the streusel topping by combining the brown sugar, flour, oats, cinnamon, and salt in a medium bowl (mixing the ingredients first by hand prevents the oats from getting chopped up too finely in the food processor). Transfer the ingredients to your food processor. Scatter in the butter, then pulse 10 times. Process continually for 10-15 seconds, only until no visible lumps of butter appear. Transfer the mix back into a medium bowl and press together to make one cohesive mass.
• Refrigerate the streusel until ready to use.
• In a large bowl, make a cornstarch slurry by whisking 1 tbsp water with the cornstarch until cornstarch is dissolved. Add the blueberries, sugar (if using), lemon zest and vanilla, and mix the ingredients until combined. Spread the mixture evenly into an 8 x 8 baking dish (or equivalent). Break up the large mass of streusel into pebble sized pieces and spread the streusel topping over the fruit. Cover the crisp loosely with foil.
• Place the crisp on the preheated sheet pan on the middle rack in oven for 15 minutes. Reduce the temperature to 350°(F), remove the foil, and bake until the topping is brown and fruit is tender, about 25-30 minutes more. 
• Serve warm or at room temperature.