Directions

• Preheat oven to 425 degrees (F). Prepare streusel topping by combining 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 ingredients to your food processor. Scatter in butter, then pulse 10 times. Process continually for 10-15 seconds, only until no visible lumps of butter appear. Transfer mix back into medium bowl and press together to make one cohesive mass.
• Refrigerate 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 blueberries, sugar, and vanilla, and mix ingredients until combined. Spread mixture evenly into an 8 x 8 baking dish (or equivalent). Break up large mass of streusel into pebble sized pieces and and spread streusel topping over fruit.
• Bake crisp on middle rack in oven for 15 minutes. Reduce heat to 350 degrees and bake until topping is brown and fruit is tender, about 25-35 minutes more. 
• Serve warm or at room temperature.