To prepare oats: Combine 2 cups blueberries, 2 tablespoons brown sugar and ⅛ teaspoon salt in a small saucepan; cook over medium heat, stirring frequently, until the blueberries are bubbling, softened and burst, about 6 minutes. Remove from heat and stir in 1 tablespoon lemon juice, 1½ teaspoons butter, ½ teaspoon vanilla and ¼ teaspoon lemon zest. Set aside to cool slightly.

Stir 2 cups oats, 2 cups oat milk and 1 tablespoon flaxseed together in a medium bowl until well combined. Spoon about ¼ cup of the oat mixture into each of 4 (8-ounce) jars. Top each jar with a generous 1½ tablespoons of the blueberry mixture. Repeat with two more layers. Cover and refrigerate for at least 8 hours (and up to 3 days).

To prepare crumble topping: Preheat oven to 375°F. Line a small rimmed baking sheet with parchment paper. Stir 2 tablespoons flour, 2 tablespoons oats, 1 tablespoon brown sugar, ¼ teaspoon cinnamon and ⅛ teaspoon salt together in a small bowl. Add 1 tablespoon butter; rub together between fingers until pea-size lumps form. Spread evenly on the prepared baking sheet.

Bake until fragrant and lightly toasted, about 5 minutes; stir. Bake until lightly browned, another 2 minutes. Cool on the baking sheet for 15 minutes. Before serving the oats, top each with about 2 tablespoons of the topping. Garnish with additional blueberries, if desired.