Heat the oven: Preheat to 375°F (190°C).
Lightly butter a small baking dish (about 8-inch square) or a pie dish.
Mix the dry ingredients: In a small bowl, whisk sugar, cornstarch, and salt until no lumps remain. This helps the thickener distribute evenly.
Toss the berries: Add blueberries to the baking dish. Sprinkle the sugar mixture over them.
Add lemon juice, lemon zest if using, and vanilla. Gently toss to coat.
Add the fat (optional): Dot the top with small pieces of butter or drizzle with melted coconut oil for extra richness.
Bake until bubbly: Place in the oven for 18–22 minutes, until the berries are soft, glossy, and releasing juices that look slightly thickened around the edges.
Rest before serving: Let the berries sit for 5–10 minutes. The sauce will thicken as it cools.
Serve your way: Spoon over vanilla ice cream, Greek yogurt, pancakes, waffles, oatmeal, or cheesecake.
Or just enjoy warm in a bowl.