Make the pastry: Combine the flour, sugar, and salt in a medium bowl. Sprinkle butter over dough and using a pastry blender or your fingertips, work it into the flour until the butter resembles small peas. Sprinkle sour cream and 2 tablespoons of the water over mixture and stir/mash it together to combine; it should form large clumps; add last 1 tablespoon water if it does not. Bring it together into a single mass and transfer dough to a medium square of parchment paper, patting it into a flatter packet, and wrap it tightly. Chill the dough until firm, 1 to 2 hours in the fridge.

Make the balsamic red onions: Heat a large skillet over medium heat and add the butter. Once melted, cook the onions, stirring, until softened and beginning to lightly brown at the edges, about 5 minutes. Add sugar, salt, balsamic, and pepper and cook, stirring, until balsamic reduces to a dark, jammy puddle, about 3 to 5 more minutes. Transfer to a bowl to cool.

To assemble: Heat oven to 400°F. Unwrap your chilled dough and reuse the parchment, if you can, to line a small baking sheet. Flour your counter and the top of the dough and roll into a large, thin circle, about 10 inches. Check for size by placing your brie in the middle and seeing if you have enough dough to cover it across the top. Once dough is the correct size, thinly spread the first teaspoon of dijon in the middle, scatter half of your red onions over the dijon, and sprinkle them with half the thyme, if using. Place brie on top. Thinly spread remaining Dijon on top of brie, followed by remaining onions and thyme. Bring sides of dough up to wrap brie, making little folds to help it fit. Press edges together gently at the top.

Transfer pastry-wrapped brie to baking sheet. Brush sides, pleats, and top of pastry with egg wash and sprinkle with sesame seeds, if using.

Bake for 30 minutes, or until golden brown all over, rotating the tray if needed for even color. If you can bear it, let rest on a cooling rack for 10 minutes.

Serve: Transfer brie to a plate with a rim (it will be runny when you cut into it) and serve with crackers or baguette slices.