Heat the oven to high broil and arrange a rack in the middle.
Place bell peppers directly on the rack. (The extra distance between the peppers and the direct heat of the broiler will cause the peppers to roast a bit before the skins blister.) Turn peppers so that the skins blacken and blister on all sides. Remove peppers from the oven and place in a heatproof bowl, cover tightly with plastic wrap, and let sit for about 30 minutes.
Place pancetta pieces and rosemary sprigs in a medium frying pan over medium heat. Cook, stirring occasionally, until pancetta is slightly rendered and just browning around the edges. Remove to a plate lined with paper towels to drain. Return the pan (with drippings) to the heat and add onion. Stir once and then leave to caramelize. Stir a few more times so it doesn’t stick to the pan. It is finished once it is golden brown and softened, about 10 minutes.
Heat the oven to 400°F and arrange a rack in the middle. When peppers are cool enough to handle, remove their skins and slice them open. Remove seeds and membranes and discard. Slice peppers into 1/4-inch-thick strips. Combine peppers with caramelized onion, season with salt and freshly ground pepper, and set aside.
Roll out pizza dough to a 14-by-10-inch rectangle and transfer to a baking sheet. Brush dough with olive oil and sprinkle with kosher salt. Distribute 1/2 of the cheese over the dough. Spread the pepper-and-onion mixture over the pizza and top with pancetta pieces. Sprinkle with remaining cheese.
Bake until the underside of the crust is golden brown, about 40 to 50 minutes.
Beverage pairing:Renato Ratti Barbera d’Alba, Italy. Pizza wine is a term used to describe something juicy, not too complex or tannic, usually with some spice or pepper to it. And those words perfectly describe this lovely, quaffable Barbera that will go down well with this pie.