Can I make the dough by hand without a mixer?

Yes. Stir with a spoon, then use the quick stretch and folds as described. This builds gluten with almost no effort.

Do I have to caramelize the onions fully?

You will get the best flavor if they are deep golden. In a rush, soften them for 10 minutes and bake. Still tasty, just less sweet.

A standard 13 by 18 rimmed sheet pan gives thinner, crisper focaccia. A 9 by 13 makes a taller slice. Grease well either way.

Absolutely. Sprinkle grated Parmesan or mozzarella in the last 5 minutes of baking so it melts without burning.

Totally. Onion Focaccia Bread is very forgiving, and the dough does most of the work while it rests.

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In a large bowl, whisk flour, yeast, and salt. Add warm water and olive oil. Stir until no dry patches remain, then let rest for 15 minutes.
Stretch the dough by wetting your hand and folding it over itself, turning the bowl and repeating 4 times. Cover and let it rise in a warm spot until puffy and doubled, about 60-90 minutes.
Cook sliced onions in a skillet with olive oil and a pinch of salt over medium-low heat until golden and sweet, about 20-30 minutes. Add balsamic vinegar at the end if desired.
Coat a rimmed sheet pan with olive oil. Transfer the dough to the pan and press it outward. Cover and rest for 20-30 minutes for a second rise.
Once puffy, drizzle more oil on top and dimple the dough with your fingertips. Scatter the caramelized onions evenly and finish with flaky salt and herbs if using.
Bake at 425°F until golden and caramelized, about 22-28 minutes. Let cool before slicing.

This focaccia can be stored at room temperature for 1 day, refrigerated for up to 4 days, or frozen for up to 2 months. Reheat to restore crispness.