How to Make It
In a standing mixer fitted with the paddle, mix the warm water with the yeast and sugar. Let stand until foamy.
Add 1/4 cup of the olive oil to the yeast mixture and combine. At low speed, add the flour and salt and mix for 2 minutes. Change to the bread hook and knead the dough at medium low speed for 8 minutes; the dough will be very sticky.
Scrape the dough into a well-oiled large bowl and brush with olive oil. Cover with plastic wrap and let the dough rise in a cool place until doubled in bulk, about 1 1/2 hours.
Using floured hands, punch down the dough and turn it out onto a lightly floured surface. Cut the dough into quarters and roll it into 4 balls. Set 2 balls of dough on each of 2 oiled cookie sheets. Brush the dough with olive oil and let it stand uncovered in a cool place for 15 minutes.
Using oiled fingertips, press and stretch each ball of dough into an 8-inch round. Dimple the rounds slightly with your fingertips; brush with olive oil. Let the dough rise in a cool place until light, about 1 hour and 15 minutes.
In a large skillet, heat 2 tablespoons of the olive oil. Add the onion, water and chopped thyme and season with salt. Cook the onion over low heat, stirring occasionally, until very soft and golden brown, about 1 hour; let cool.
Set a pizza stone in the oven and preheat to 450°; the stone will be fully heated in about 30 minutes. Using a mandoline, thinly slice the fingerling potatoes. In a bowl, toss the potatoes with a little olive oil, salt and pepper and the thyme sprigs. Dimple the dough again and top with the caramelized onion and the potatoes.
Slide 2 of the focaccias onto the pizza stone and bake for 12 to 15 minutes, or until golden brown and puffed around the edges. Remove the focaccias from the oven, top them with dollops of Robiola cheese and drizzle with olive oil. Return them to the oven for 1 minute to warm the cheese. Repeat with the 2 remaining focaccias. Serve hot.