Adjust oven rack to lowest position and heat oven to 250 degrees. Pat chicken dry with paper towels and season with salt and pepper. Heat oil in large Dutch oven over medium heat until just smoking. Add chicken breast-side down; scatter onion, celery, garlic, bay leaf, and rosemary around chicken. Cook until breast is lightly browned, about 5 minutes. Using a wooden spoon inserted into cavity of bird, flip chicken breast-side up and cook until chicken and vegetables are well browned, 6 to 8 minutes. Remove Dutch oven from heat; place large sheet of foil over pot and cover tightly with lid. Transfer pot to oven and cook until instant-read thermometer registers 160 degrees when inserted in thickest part of breast and 175 degrees in thickest part of thigh, 80 to 110 minutes.
Transfer chicken to carving board, tent with foil, and rest 20 minutes. Meanwhile, strain chicken juices from pot through fine-mesh strainer into fat separator, pressing on solids to extract liquid; discard solids (you should have about ¾ cup juices). Allow liquid to settle 5 minutes, then pour into saucepan and set over low heat. Carve chicken, adding any accumulated juices to saucepan. Stir lemon juice into jus to taste. Serve chicken, passing jus at table.
NOTES (from Cook’s): The cooking times in the recipe are for a 4½ - to 5-pound bird. A 3½ - to 4½ -pound chicken will take about an hour to cook, and a 5- to 6-pound bird will take close to 2 hours. Cook’s developed this recipe to work with a 5- to 8-quart Dutch oven with a tight-fitting lid. If using a 5-quart pot, do not cook a chicken larger than 5 pounds. Use the best chicken available, such as a Bell & Evans. If using a kosher chicken, reduce the kosher salt to 1 teaspoon (or ½ teaspoon table salt). If you choose not to serve the skin with the chicken, simply remove it before carving. The amount of jus will vary depending on the size of the chicken; season it with about ¼ teaspoon lemon juice for every ¼ cup.
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