Put the water into a large bowl, or the bowl of your stand mixer. Sprinkle over the yeast and sugar and let stand for 5-10 minutes, until it gets foamy. (If it doesn't, you may need fresh yeast.)
Add the flour, olive oil and salt and stir until the dough comes together. Knead by hand or use the dough hook of your stand mixer until the dough is relatively smooth, but still tacky. Shape it into a rough ball, drizzle with oil and let sit in the bowl, covered with a tea towel, for an hour or two. (If you like, put it in the fridge overnight after it rises for awhile at room temperature-the cold will slow the rise.)
When you're ready to bake, preheat the oven to 425F. Drizzle some oil into a 9 or 10-inch cast iron skillet or other ovenproof pan or dish and press the dough out about an inch thick in the pan. Poke deeply all over the surface of the dough with your fingers-this will create a nubbly surface for the oil and seasonings to settle into. Sprinkle with finely chopped fresh rosemary (about a teaspoon) or everything bagel seasoning (a couple teaspoons), or anything else you like on your focaccia. Drizzle with olive oil and sprinkle with flaky salt. (Don't use much if you've already added everything bagel seasoning, which should contain salt.)
Bake for 15-20 minutes, until deep golden. Serve in wedges. Serves 8-12.