How to Make It
Place water, yeast, and sugar in bowl of a heavy-duty stand mixer fitted with a dough hook attachment; whisk together by hand. Let stand until foamy, about 5 minutes. Add flour and kosher salt; beat on low speed until dough comes together to a slightly sticky consistency, about 1 minute. Increase speed to medium-low, and beat until smooth and elastic, about 6 minutes. Coat a large bowl with 1 tablespoon of the oil. Place dough in oiled bowl, and cover with plastic wrap. Let rise in a warm place (about 85°F) until doubled in size, about 1 hour, 30 minutes.
Coat a 17- x 12-inch rimmed baking sheet with 3 tablespoons of the oil. Remove dough from bowl; stretch dough to fit inside prepared baking sheet, pressing with fingers so dough fits into all corners. Cover with plastic wrap; chill at least 8 hours or up to overnight.
Remove dough from refrigerator. Let rise, covered, in a warm place (about 85°F) until very puffed and almost doubled in height, about 1 hour, 30 minutes.
Meanwhile, preheat oven to 425°F with rack in upper third position. Place garlic, rosemary sprigs, thyme sprigs, and remaining 1/2 cup oil in a small saucepan. Cook over medium, undisturbed, until fragrant, about 10 minutes. Remove from heat; let cool at least 15 minutes or until dough has finished rising. Strain oil; discard solids.
Uncover risen dough. Using fingers, lightly press indentions into top of dough. Pour cooled oil all over indented dough. Sprinkle evenly with flaky sea salt and pepper.
Bake on upper third rack in preheated oven until focaccia is golden brown, 20 to 22 minutes. Remove from oven; garnish with rosemary leaves and thyme leaves. Let cool on baking sheet 10 minutes. Remove from baking sheet, and transfer to a wire rack; let cool completely, about 30 minutes. Cut evenly into 20 pieces.