Heat 3 tablespoons butter and 1½ tablespoons olive oil in a large, deep skillet over medium heat.

Add the sliced onions and a pinch of salt. Cook, stirring occasionally, for 5 minutes.Tip: Low and slow is key! If the onions are browning too quickly, reduce the heat. Stir occasionally to prevent burning. (Note 5)Lower the heat to medium-low and continue cooking for 35–40 minutes, stirring occasionally, until the onions are golden brown and caramelized.Check at 30 minutes and adjust heat as needed to avoid burning. (Note 6)Shortcut: Add ½ teaspoon of sugar to help the onions brown faster. (Note 7)

Stir in the 1½ tablespoons balsamic vinegar and cook for another 2 minutes.Remove the onions from the skillet and set aside.

In the same skillet, increase heat to medium-high and add a little more oil if needed (about 1 teaspoon at a time).

Season the 6 boneless, skinless chicken thighs with 1½ teaspoons garlic powder, 1½ teaspoons dried thyme, Salt and pepper.Tip: Let the chicken thighs sit at room temperature for 10 minutes before cooking for even searing. (Note 8)

Sear the chicken for 2–3 minutes per side until golden brown but not fully cooked. Remove and set aside.Tip: Don’t overcrowd the pan - cook in batches if needed to get a proper sear. (Note 9)

Reduce heat to medium.Sprinkle 1 tablespoon all-purpose flour into the skillet and stir for 1 minute to cook out the raw flour taste.Slowly whisk in 1¼ cups beef broth, stirring constantly to prevent lumps.Tip: If the sauce thickens too much, add a splash of extra broth to loosen it up. (Note 10)Simmer for 1–2 minutes until the broth slightly thickens.

Return the caramelized onions to the skillet, spreading them evenly in the sauce.

Nestle the seared chicken thighs into the onions and broth.Cover with a lid and let simmer on medium-low heat for 15 minutes, or until the chicken reaches an internal temperature of 165 F (74 C). (Note 11)

Once the chicken is fully cooked, place 1 slice of Swiss cheese on top of each piece.Cover the skillet again and let it cook for 2–3 more minutes, until the cheese is melted and gooey.Tip: Turn off the heat and let the residual warmth melt the cheese if you prefer a softer melt. (Note 12)

Garnish with fresh parsley, if desired.Serve with garlic mashed potatoes, rice, or crusty bread to soak up the rich sauce.