Combine 180 g flour, 12.5 g sugar, and salt. Cut in cold butter with a pastry blender or fingertips until the mixture has pea-sized pieces. Add 4 tablespoons ice water and toss until clumps hold together when squeezed; add up to 1 tablespoon more only if needed. Press into a disk, wrap, and refrigerate 45 minutes.

Heat oven to 400°F. Wash and dry pears, remove stems, cores, and seeds, then slice about 1/8 inch thick. Weigh 500 g prepared fruit. Toss gently with 25 g sugar, 12 g flour, lemon juice, and cinnamon just before assembling.

Roll dough between two sheets of parchment into a 12-inch circle. Remove the upper sheet and slide the lower parchment and dough onto a large rimmed baking sheet. Arrange pears evenly in the center, leaving a 2-inch border; leave excess pooled liquid in the bowl. Fold the border over the fruit in overlapping sections, pressing cracks closed.

Brush the exposed crust with milk and sprinkle with the remaining 12.5 g sugar. Refrigerate the assembled galette 15 minutes. Bake 40–45 minutes, until crust is deeply golden, bottom is baked through, and a knife slips easily into the pears. If edges brown early, cover just the edges loosely with foil.

Cool on the baking sheet 20 minutes before cutting into 8 wedges. Refrigerate covered leftovers within 2 hours and use within 3 days.