Whisk together the flour, salt, and yeast in a large bowl. Pour in the ice water, stirring as you pour and mixing until thoroughly incorporated. Add 1 tbsp olive oil and stir some more. The dough should be slightly stiff but still wet enough to stir and slightly sticky. Brush the top with a little olive oil and cover the bowl with plastic wrap. Refrigerate the dough for 5 hours. After that remove the bowl from the refrigerator and let the dough rise, covered, at room temperature for 12 hours.
Generously coat a round baking pan with olive oil. Put the dough into the pan, drizzle 1 tbsp olive oil over the top, press the dough out to the rim of the pan. Cover the pan loosely with plastic wrap. Refrigerate the dough for 3 hours. Then remove the pan from the refrigerator and let it rise at room temperature until puffy and almost doubled size
Prepare the toppings while the dough is in its final rise. If using pancetta, put the pancetta cubes in a dry nonstick skillet over medium heat. When the pancetta begins to sizzle, lower the heat to medium-low and sauté, stirring from time to time, for 10 minutes.
Combine the rosemary and the 2 tbsp olive oil in a small bowl. Let the oil sit until the dough is ready.
Place an oven rack in the lowest position in the oven and heat the oven to 475 F/240 C/gas 9.
When the dough has risen, remove the plastic wrap. Arrange the cherry tomatoes on top of the dough, pressing them in lightly. Then press in the diced onion, and finally the pancetta cubes. Gently brush the rosemary-oil mixture over the top of the dough and sprinkle with sea salt.
Bake the focaccia for 30 minutes,
Set the pan on a wire rack to cool for 10 minutes.