2 cups plain flour
2 tsp. salt
120g chilled unsalted butter, cut into pieces
1 Tbsp. apple cider vinegar
700g tomatoes, sliced
3 garlic cloves, thinly sliced
1 cup firm cheese (cheddar, or Gouda), finely grated
1 large egg, beaten
Sea salt & ground black pepper
1 Tbsp. finely chopped chives
Pulse flour and 1¼ tsp. salt in a food processor to combine. Add butter and pulse until mixture resembles coarse meal with a few pea-size pieces of butter remaining.
Transfer mixture to a large bowl; drizzle with vinegar and ¼ cup ice water. Mix with a fork, adding more ice water by the tablespoonful as needed, just until a shaggy dough comes together. Turn out onto a work surface and lightly knead until no dry spots remain (be careful not to overwork). Pat into a disk and wrap in plastic. Chill at least 2 hours.
Preheat oven to 200°C. Gently toss tomatoes, garlic, and remaining salt in a large bowl. Let sit 5 minutes (tomatoes will start releasing some liquid). Drain tomato mixture and transfer to paper towels.
Roll out dough on a lightly floured sheet of baking paper to a 35cm round about 3mm thick. Scatter cheese over dough, leaving a 4cm border. Arrange tomatoes and garlic over cheese. Bring edges of dough up and over filling, overlapping as needed to create about a 4cm border; brush dough with egg. Sprinkle tomatoes with sea salt and pepper. Chill in freezer 10 minutes.
Bake galette, rotating once, until crust is golden brown and cooked through, 55–65 minutes. Sprinkle with chives.