Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Toss the halved jalapeños with olive oil and kosher salt.
Arrange them cut-side up on the baking sheet.
Roast for 8–10 minutes until just tender.
Fill each pepper generously with diced brie cheese.
Return to the oven and bake another 5–7 minutes until the brie is melted and bubbly.
Meanwhile, combine 3/4 cup blueberries, sugar, and lemon juice in a saucepan.
Simmer for 8–10 minutes, gently mashing the berries until thick and glossy.
Remove from the heat and stir in the remaining blueberries.
Spoon the warm blueberry mixture over each Blueberry Brie Jalapeño Popper.
Garnish with fresh thyme leaves and black pepper flakes.
Serve warm while the cheese is perfectly gooey.