Season the chicken: In a medium bowl, combine the cubed chicken with 1/4 teaspoon of salt and 1/8 teaspoon of black pepper. Set aside.
Tip: Seasoning the chicken beforehand allows the flavors to penetrate, resulting in a more flavorful and juicy final product.
Sauté the aromatics: Heat the olive oil in a large skillet over medium-high heat. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute, until fragrant. (If using mushrooms and bell peppers, add them now and cook until softened, about 5 minutes).
Tip: Don't overcrowd the pan. If necessary, cook the vegetables in batches to ensure even browning. Watch for the onions to become translucent and the garlic to release its aroma - this signals they're ready.
Brown the chicken: Add the seasoned chicken to the skillet and cook until browned on all sides, about 5-7 minutes.
Tip: Don't move the chicken around too much while it's browning. Let it sear for a few minutes per side to develop a nice crust. This will also help keep the chicken juicy.
Simmer in the sauce: Pour in the chicken broth, heavy cream, Parmesan cheese, parsley, thyme, remaining salt and pepper, and lemon juice (if using). Bring the mixture to a simmer, then reduce heat to low, cover, and cook for 5-7 minutes, or until the chicken is cooked through and the sauce has thickened slightly
Tip: The sauce will thicken as it simmers. If it becomes too thick, add a tablespoon or two of water to adjust the consistency. Taste and adjust seasoning as needed. The internal temperature of the chicken should reach 165°F (74°C).
Serve: Serve immediately.