In a saucepan set over medium-high heat, combine the blueberries, sugar and 1/3 cup (75 mL) lemon juice and bring to a boil. Reduce heat to simmer and cook until berries are tender, about 10 min. Whisk cornstarch with 2 tbsp (30 mL) hot water until smooth and stir into blueberry mixture. Bring to a boil again and cook until thickened, about 1 min. Transfer to large bowl and cool, 10 min. Press plastic wrap onto the surface of blueberry mixture to prevent a skin from forming and chill until cold, 1 hour.
Preheat oven to 350°F (180°C). Line an 8-in. (2 L) square baking pan with parchment paper. Set aside.
In a bowl, stir together the oats, flour, brown sugar, 1 tbsp (15 mL) lemon zest and salt. Using a pastry cutter or 2 knives in a criss-cross motion, cut in butter until crumbly. Press 2/3 of the mixture into prepared pan. Spread cold blueberry mixture overtop. Sprinkle with remaining mixture. Bake until golden, about 45 minutes. Cool in pan before cutting.
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