In a medium bowl, whisk together 2 cups (254 grams) flour and ¼ teaspoon yeast. Add 1 cup (240 grams) room temperature water; stir and fold until well combined. (Mixture will start out very thick but will loosen as it sits.) Cover with plastic wrap, and let stand at room temperature for 8 to 12 hours.

In the bowl of a stand mixer fitted with the paddle attachment, beat kosher salt, remaining 4 cups (508 grams) flour, and remaining 1 teaspoon (3 grams) yeast at low speed until combined. Add flour-yeast mixture and 1 cup (240 grams) warm water; carefully pulse mixer between lowest speed and “off” setting until mixture is just combined. Beat at low speed until a rough, sticky dough forms, 1 to 2 minutes, stopping to scrape sides of bowl.

Switch to the dough hook attachment. Beat at low speed until a soft, tacky dough forms and cleans sides of bowl, 10 to 12 minutes. (Dough should pass the windowpane test; see Note.)

Spray a large bowl with cooking spray. Place dough in bowl, turning to grease top. Cover with plastic wrap, and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, about 1 hour.

Line 2 baking sheets with parchment paper.

Punch dough down; cover and let stand for 10 minutes. On a clean surface, divide dough into 12 portions (about 100 grams each). Gently shape each portion into a ball; cover and let stand for 15 minutes. Press each ball into a 4-inch-wide round; place 1½ to 2 inches apart on prepared pans, reshaping rounds as needed. Using your fingers, press a 2-inch-wide well in center of each round until bottom of well is completely flattened. Cover with greased sheets of plastic wrap; let rise in a warm, draft-free place (75°F/24°C) until puffed, 30 to 40 minutes.

Preheat oven to 425°F (220°C).

In a small bowl, combine all seeds and sea salt.

Using your fingers, press and reshape wells again. Using the tines of a fork, dock bottom and sides of wells 5 to 6 times; fill each well with 1 tablespoon (20 grams) Caramelized Onion Filling, spreading into an even layer. Brush exposed dough edges with egg, and sprinkle with seed mixture.

Bake until lightly golden, 12 to 15 minutes, loosely covering with foil during final 2 to 3 minutes of baking to prevent excess browning. Let cool on pan for 15 minutes; serve warm.