Garlicky Fried Chicken

Purée onions and garlic with 1/3 cup salt, 1 tablespoon black pepper, cayenne, and 1 cup water in a food processor until smooth, then pour into a large bowl and stir in remaining 7 cups water. Divide chicken and marinade among 3 (1-gallon) sealable plastic bags, then seal, forcing out excess air. Marinate chicken, chilled, turning bags over a few times, at least 12 hours. (If you're concerned about leaks, put bags in a large bowl.)

Whisk together flour, remaining 2 teaspoons salt, and remaining tablespoon black pepper in a large bowl, then divide between 2 large roasting pans.

Drain chicken in a colander, discarding marinade, then dredge each piece of chicken in seasoned flour and leave sitting in flour.

Let chicken stand 20 minutes.

Preheat oven to 250°F. Line 2 large baking sheets with paper towels and set a cooling rack on each.

Heat 2 inches oil in pot over moderately high heat until it registers 365°F on thermometer. Knocking off excess flour, transfer 4 pieces chicken to oil (keeping batches all white meat or all dark) and fry, turning occasionally with tongs, until golden brown and cooked through, about 8 minutes for breasts or 10 to 12 minutes for drumsticks and thighs. Lift chicken from oil with tongs, letting excess oil drip back into pot, then transfer to a rack and keep warm in oven. Dredge and fry remaining chicken in same manner, keeping it warm in oven. (Return oil to 365°F between batches.)

Chicken can marinate up to 1 day.