Place the onions and garlic on the bottom of a 13-inch by 9-inch baking pan.
In a small bowl, combine the paprika, thyme, rosemary, and pepper. Coat the chicken with the oil and sprinkle the paprika mixture evenly over the chicken. Place on top of the onions and garlic.
Bake 60–70 minutes or until internal temperature reaches 165°F when tested with a meat thermometer.
Remove the chicken with 2 large spoons, preferably slotted, and place on cutting board. Let stand 10 minutes before slicing, discarding the skin.
Meanwhile, to “degrease” the pan drippings, add the cold water to the pan drippings. Stir until well blended and drain the pan residue through a fine-mesh sieve, reserving both the liquid and the strained onion mixture.
Pour the liquid portion into a resealable plastic sandwich bag, seal tightly, and place in freezer for 15 minutes to allow the fat to rise to the top.
Hold the bag over a small saucepan. Using scissors, cut about ¼ inch off of the corner of the bag, allowing the juices to pour into the saucepan. Stop the flow to prevent the fat from being released. Discard the bag with the fat. Stir in the cornstarch and salt until cornstarch is completely dissolved.
Add the reserved onion mixture. Bring to a boil over medium-high heat. Boil 1 minute, stirring frequently. Serve with the sliced chicken.