'1 (1/4-ounce) envelope active dry yeast (about 2 1/4 teaspoons)'
'Pinch of sugar'
'3 1/2 cups all-purpose flour
divided
plus more for surface'
'2 teaspoons kosher salt
plus more'
'6 tablespoons olive oil
divided
plus more'
'2 small red waxy potatoes (about 1/2 pound)
scrubbed
very thinly sliced'
'1 red onion
very thinly sliced'
'1 medium zucchini
very thinly sliced'
'4 ounces Fontina cheese
grated (about 1 1/2 cups)'
'Flaky sea salt'
'Coarsely ground black pepper'
Whisk yeast, sugar, and 3 Tbsp
warm water in a large bowl
Let sit until surface is foamy, about 5 minutes
Add 1 1/4 cups room-temperature water, then add 1 cup flour, whisking until smooth (mixture should be the consistency of pancake batter)
Add 2 1/2 cups flour and 2 tsp
kosher salt and mix with a wooden spoon until a shaggy dough forms
Turn out dough onto a lightly floured surface and knead with your hands, adding more flour as needed to prevent it from sticking, until dough is smooth and supple, 5–10 minutes (it will be very soft)
Transfer to a large oiled bowl and cover with plastic wrap
Let rise in a warm, draft-free spot until doubled in size, 1–1 1/2 hours
Divide dough into 4 equal pieces and form into balls
Place on an oiled rimmed baking sheet and cover with plastic wrap; let rise in a warm, draft-free spot until balls of dough are nearly doubled in size, 40–50 minutes
Meanwhile, place potato slices in a colander and rinse with cold water
Drain, season with kosher salt, and let sit, tossing occasionally, until softened, 10–15 minutes
Transfer to a bowl and toss with onion, zucchini, and 2 Tbsp
oil; set aside
Preheat oven to 450°F
Working with 1 ball of dough at a time and keeping remaining dough covered while you work, roll out dough on a lightly floured surface into 10" rounds
Heat a dry large cast-iron skillet over medium
Pour 1 Tbsp
oil into skillet and cook a dough round until bottom is golden brown and top looks dry, about 1 minute
Flip and cook just until bottom is lightly golden, about 1 minute
Transfer to a rimmed baking sheet and repeat with remaining 3 Tbsp
oil and rounds of dough (you should be able to fit two pizzettas per baking sheet if you overlap them slightly)
Top pizzettas with Fontina cheese and reserved potato mixture; season with kosher salt
Bake, rotating baking sheets once top to bottom and back to front, until crusts are golden brown and crisp and cheese is bubbling, 10–15 minutes
Remove pizzettas from oven, season with sea salt and pepper, and drizzle with oil