Preheat a large ovenproof skillet over medium heat.
While the skillet heats up, pat the chicken thighs dry with paper towels. Season with salt and pepper on both sides.
2 lb chicken thighs, boneless skinless
½ tsp salt
¼ tsp black pepper
Once the skillet is hot, add butter and swirl to coat the bottom. Add chicken to the skillet and pan-fry until golden brown and cooked through, 3-4 minutes per side. Transfer to a plate and loosely cover with aluminum foil. (Reserve skillet with drippings.)
2 tbsp butter, unsalted
Meanwhile, wash and dry the fresh produce.
1 lb white mushrooms
1 small pkg fresh thyme
1 small bunch Italian (flat-leaf) parsley
Peel, halve, and thinly slice onions into half-moons.
2 medium yellow onions
Return skillet to medium heat. Add more butter and swirl to coat the bottom. Add onion and salt; cook, stirring occasionally, until soft and caramelized, 15-20 minutes.
¼ cup butter, unsalted
½ tsp salt
While the onions cook, peel and mince garlic; place in a medium bowl.
4 cloves garlic
Thinly slice mushrooms and add to the bowl with the garlic.
Slide thyme leaves off the stems; discard the stems and add leaves to the bowl.
Shave parsley leaves off the stems; discard stems and mince the leaves.
When the onions are done, add garlic, mushrooms, thyme, half of the parsley (reserving the rest for garnish), broth, and Worcestershire to the skillet; stir to combine. Bring to a boil, then reduce heat to a simmer and cook, stirring occasionally, until liquid is reduced by half, 4-5 minutes.
Preheat oven to broil on high and position rack in the upper third of the oven.
When the veggies are done, nestle chicken into the mushrooms; top chicken with mozzarella and Parmesan. Place skillet in the oven and broil until cheese is melted and bubbly, 3-4 minutes. Remove from oven.
To serve, divide veggies and chicken between bowls or plates. Top with reserved parsley and enjoy!