Classic focaccia is the perfect canvas for a beautiful and edible fall design. The base is a tender airy dough that's delectably chewy. Topped with oyster mushrooms, herbs and other vegetables arranged like foliage, the result is a delicious bread as well as a striking centerpiece for your table.
For the dough: Whisk together the flour, yeast and sugar in the bowl of a stand mixer fitted with a dough hook. Stir in the warm water and 2 tablespoons of the olive oil while the machine is on low speed, mixing until the flour is completely moistened. Let stand 5 minutes.
Add the salt and knead on medium speed for 5 minutes. (The dough will tighten up, then begin to relax. After 5 minutes, it will be very wet and stick to the bottom of the mixer but not the sides.)
Rub 2 tablespoons olive oil around the bottom and sides of a large mixing bowl (large enough to hold at least double the volume of dough). Transfer the dough to the bowl with a scraper or spatula, flipping once to coat completely with oil. Cover with plastic wrap and let stand until doubled in size, about 2 hours.
Pour the remaining 1/4 cup olive oil into the bottom of an 18-by-13-inch rimmed baking sheet. Use your hands to evenly distribute the oil along the bottom and up the sides. Scrape the dough into the baking sheet, flattening it slightly with your hands. Flip the dough once and gently stretch it to fit into an even layer in the baking sheet. (The dough may not stretch to the edges at this point. If it starts to pull back, let rest 10 minutes and re-stretch, making sure to pull from the center as well as the edges to get an even thickness.) Use your fingertips to make dimples over the surface of the focaccia, pressing firmly but not so hard you make holes in the dough.
For the brine and toppings: Dissolve the kosher salt in 1/3 cup warm water. Drizzle over the surface of the focaccia, letting it pool in the dimples.
Starting in the upper right corner, gently press the largest mushroom pieces into the dough and continue down and to the left, working from the largest pieces to the smallest in a curling, tapering wave shape. Fill in the spaces between the mushrooms with the arugula, radicchio, carrot slices, fennel, fennel fronds and rosemary sprigs. Let stand in a warm place for 45 minutes so the dough can relax and rise again.
Meanwhile, preheat the oven to 450 degrees F.
Sprinkle the focaccia all over with flaky sea salt and sprinkle the vegetables with a couple pinches of red pepper flakes if using. Bake, rotating the baking sheet halfway through, until the focaccia is deep golden brown on the top and bottom, 30 to 35 minutes. Drizzle with the olive oil and let rest for 5 minutes. Use a spatula to remove the focaccia to a wire rack to cool completely.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)