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.