In a large bowl, combine the flour, 1 teaspoon kosher salt, and instant yeast. Add the water and use a spoon, rubber spatula or a dough whisk, mix until the water is absorbed and a shaggy, sticky dough is formed. Cover with a towel or plastic wrap and let rise until doubled at room temperature for 1 3/4 to 2 hours. Alternatively, you can let it rise in the fridge overnight for 8 to 10 hours.

Heat a large sauté pan over medium heat. Once hot, add 2 tablespoons olive oil. Once the oil is heated, add the onions and 1 teaspoon kosher salt. Cook onions, stirring every minute or two, until a medium brown, almost caramel colored, about 25 minutes. Scrape onions onto a plate to cool.

When the dough is doubled, line a 9×13 cake pan with parchment paper and drizzle 2 tablespoons olive oil over it. Do not deflate your dough, just scrape it onto the oiled parchment. Drizzle the top of the dough with another tablespoon of olive oil and use your fingers to dimple the dough, flattening it out. Let the dimpled dough rest at room temperature for 15 minutes and heat your oven to 425°F. After 15 minutes, dimple the dough only where needed a little further into the corners. Let rest for a final 15 minutes before scattering the top with onions, poppy seeds, and a few pinches of salt.

Bake the focaccia for 25 minutes, until deeply golden brown at the edges and across the top. While it bakes, you can prepare any toppings you’d like to serve it with, such as cream cheese or butter, lox, thinly sliced tomatoes, cucumbers, onions, or capers.

Loosen the focaccia if it’s stuck in any place and slide it into a cutting board. Cut into 12 squares, using a sharp knife to get through the onions on top without pulling them off, and replacing any that scatter. Eat right away.

Focaccia keeps at room temperature for 1 to 2 days. Reheat on a baking sheet at 350°F for 10 to 12 minutes.