The next day, in a large bowl, whisk the lukewarm water with the yeast, stirring to dissolve. Add your biga, using a spatula to mix and dissolve it into the liquid as much as you can. Mix in the olive oil. Add the flour and salt, mixing with a rubber spatula until a sticky dough forms and no dry streaks remain. Cover the bowl and let it rest for 30 minutes.
Stretch and fold: Wet your hand and grab an edge of the dough; pull it up and stretch it towards the center. Repeat this process, stretching and folding 8-10 times, rotating the bowl with each pull. As you go, you will notice the dough getting smoother and developing more structure. Watch this video for guidance. Overnight Proof: Rub the surface of the dough lightly with olive oil. Cover the bowl with a lid or plastic wrap and place it in the refrigerator for at least 12 hours and up to 3 days.
Prep the pan and deflate: Generously grease your baking sheet with butter, then drizzle the center with olive oil. Take your cold dough out of the fridge. Using two forks, deflate the dough by releasing it from the sides of the bowl and pulling it toward the center, just like you did during the stretch and fold process. Do this four times, rotating the bowl with each fold (technique from Alexandra's Kitchen). Place the dough ball onto the prepared baking dish and coat it with olive oil. Allow it to rest in the pan, uncovered, until it has doubled in size, at least 2 hours and up to 4 hours. The dough is ready when you poke it with a finger and it springs back slowly, leaving a small, visible indentation. If it springs back quickly, the dough isn't ready yet.
Set a rack in the middle of the oven and preheat it to 425°F (220°C).
Lightly oil your hands. Using your fingers, gently press straight down into the dough to create deep dimples while simultaneously nudging and stretching the dough out to fill the corners of the pan. Drizzle the surface with more olive oil, sprinkle it with flaky sea salt, and top with the sesame seeds.
Bake until puffed and golden brown on top, and the underside is crisp and golden, about 20-30 minutes.
Let the focaccia cool, then slice it into small rectangles. Serve with whipped butter and sliced radishes, if desired.