Gluten-Free Focaccia With Rosemary and Onion

Stir 2 Tbsp. psyllium husk, 4 tsp. Diamond Crystal or 2½ tsp. Morton kosher salt, 2 tsp. sugar, and 2 cups room-temperature water in the bowl of stand mixer to combine; let sit until thickened slightly, 5–10 minutes.

Add 2¼ tsp. instant yeast, 1½ tsp. baking powder, and 2 tsp. finely chopped rosemary to mixture and stir vigorously until no large, undissolved lumps remain. Add 3 cups (375 g) gluten-free flour blend, 2 tsp. apple cider vinegar, and ¼ cup extra-virgin olive oil; fit onto stand mixer and attach dough hook. Mix on low speed until dough is no longer shaggy (it should be sticky but smooth), 5–8 minutes. Remove mixing bowl from stand mixer; cover tightly with a plate or plastic wrap. Let sit in a warm, draft-free place until almost doubled in size, 1–1½ hours.

Generously grease a 10"-diameter cast-iron skillet or 9" square metal pan with room-temperature unsalted butter; pour 2 Tbsp. extra-virgin olive oil into pan and swirl to coat (this double greasing prevents any sticking and helps create a golden brown bottom crust). Using a rubber spatula, scrape focaccia dough into prepared pan. With oiled hands, pat dough to smooth surface and extend to edges of pan. Cover (a baking sheet is excellent for this) and let dough rise in a warm place until puffy and jiggly, 45–60 minutes, depending on the temperature of the room.

Place a rack in middle of oven; preheat to 425°. Scatter 1 medium red onion, cut into ¼"-thick wedges, and rosemary leaves over the top of the dough, pushing gently into focaccia to adhere. Drizzle remaining 1 Tbsp. extra-virgin olive oil over and season lightly with flaky sea salt and freshly ground pepper. Bake focaccia, covering loosely with foil if onions are browning too quickly, until puffed and deep golden brown and an instant-read thermometer inserted into the center registers 205°–210°, 50–60 minutes. Let cool 15 minutes before cutting into wedges or strips.