Heat oven to 350°F Cut round of parchment paper to fit diameter of Dutch oven. With kitchen shears, remove back rib bones from breast and thigh portions of chicken, (This will make the pieces smaller and easier to fit into the pot.) Stir butter, thyme, poultry seasoning, salt and pepper in small bowl. Coat chicken with half of the butter mixture.
Heat Dutch oven over medium-high heat until hot. Add oil; heat until hot. Cook chicken, in batches, 5 to 7 minutes or until golden brown, turning once and adjusting heat as necessary. Remove chicken.
Add onions to Dutch oven; cook and stir over medium heat 1 minute. Add garlic; cook and stir 30 seconds or until fragrant. add sweet potatoes; cook and stir 1 minute. Stir in remaining butter mixture until melted; cook 1 minute. Nestle chicken in one layer in vegetables. Press parchment paper round over chicken; cover with lid.
Bake 50 to 60 minutes or until chicken is no longer pink in center and vegetables are tender. Remove and discard parchment paper. Remove chicken and vegetables.
Skim off any fat that rises to surface of sauce; bring sauce to a boil over high heat. Stir in lemon juice; boil 3 to 4 minutes or until slightly thickened. Serve chicken alongside vegetables; spoon sauce over chicken.