Preheat oven to 425F and set rack to top third position in the oven.
In a small bowl, whisk together salt, pepper, and paprika. In a separate bowl, combine beef stock, Worcestershire sauce and sherry. Set aside.
Thoroughly pat chicken dry. Separate skin from thigh meat by inserting your index finger between the two and lifting up gently.
Season both sides of chicken, as well as underneath the skin.
Add one tablespoon oil to heavy oven-safe skillet and heat over medium-high. Once hot, add the seasoned chicken, skin side down, and sear for 5-6 minutes. Flip to the other side and continue cooking for 3-4 minutes. Remove from pan and place on plate.
Add butter to the pan and once foaming subsides, add onions, sugar, and a couple generous pinches of salt. Reduce heat to medium and cook onions until translucent, about 4-5 minutes, stirring occasionally. Add in garlic and thyme, and cook for an additional minute.
Pour in stock mixture. Replace chicken on top of onions. Place the skillet in the oven and cook for about 25 minutes, until chicken is cooked to 160F and onions are caramelized and saucy. Remove skillet from oven and add shredded Gruyere on top of chicken thighs and garnish with parsley. Continue cooking for about 5-6 minutes until cheese is melted.
Serve immediately with crusty bread to sop up he delicious onions.