On a baking sheet, toast pecans for about 10 minutes at 350 degrees, until fragrant and just beginning to darken, tossing once. Let cool.

In a food processor, process pecans until they're the consistency of coarse meal. Add flour, sugar, salt, and cinnamon to the food processor and pulse just to combine.

Add chilled butter pieces and pulse until the mixture resembles coarse meal with a few pea-size pieces.

Transfer dough mixture to a large bowl and drizzle 4 T of ice water over the Dough. Mix, adding another tablespoonful of water if needed, just until mixture comes together.

Form the dough into a 6"-diameter disk. Wrap the disc in plastic and place in the refrigerator to chill for at least 1 hour.

Do Ahead: Dough can be made 2 days ahead. Keep chilled, or freeze up to 1 month.

Preheat oven to 375 degrees.

In a large bowl, toss together blueberries, cornstarch, lemon juice, and ¼ C sugar.

Lightly flour a surface and roll out dough into a 12" round. Carefully transfer dough round to a parchment-lined baking sheet.

Mound blueberries in the center of dough, making sure to leave a 2" border.

Fold edges over of the crust over, overlapping slightly onto the blueberries.

Brush edges dough with milk and sprinkle with sugar.

Bake galette at 375 degrees until the crust is dark golden brown and the filling is bubbling, about 45–50 minutes.

Let cool before serving.

Make Ahead: Dough can be made 2 days ahead and kept chilled (or in freezer for one month). Galette can be baked 1 day ahead, so long as it's tightly wrapped and kept at room temperature.