Step 1

Cook onions in a large pot of boiling salted water until tender, 5-8 minutes. Drain and let cool. Trim root ends; peel.

Step 2

Meanwhile, heat oil in a large heavy pot over medium heat. Add pancetta to pot and cook, stirring occasionally, until fat is rendered and pancetta is brown, 8-10 minutes. Using a slotted spoon, transfer pancetta to a large bowl.

Step 3

Add onions to same pot and cook, stirring occasionally, until beginning to brown, 8-10 minutes. Add garlic and cook, stirring often, until fragrant, about 3 minutes. Transfer onions and garlic to bowl with pancetta.

Step 4

Season chicken with salt and pepper. Working in batches, add chicken to pot skin side down and cook, turning, until browned on all sides, 10-15 minutes per batch; transfer to bowl with onions.

Step 5

Carefully drain fat from pot and return to medium-high heat. Add both vinegars to pot and bring to a boil, stirring and scraping up any browned bits from bottom of pot. Add broth, raisins, bay leaves, and reserved chicken, pancetta, onions, and garlic to pot. Bring to a boil, reduce heat, and simmer, partially covered, until chicken is fork-tender, 35-40 minutes.

Step 6

Using a slotted spoon, transfer chicken and onions to a large platter. Skim fat from cooking liquid and discard. Remove bay leaves, and season sauce with salt and pepper. Spoon sauce over chicken and onions.