Pat chicken breasts dry with paper towels. If breasts are thick, slice each horizontally to make two thinner cutlets (about 1/2" thick) so they cook evenly. Season both sides with 1/2 tsp salt and 1/2 tsp freshly ground black pepper.

Mince and chop

Peel and finely mince 4 garlic cloves. Peel and thinly slice the small yellow onion. Clean mushrooms with a damp towel and slice them. Core, seed and thinly slice the red bell pepper. Chop parsley and set aside.

Cook Chicken

Sear the chicken

Heat 1 tablespoon olive oil and 1/2 tablespoon butter in a large skillet over medium-high heat until shimmering. Add the seasoned chicken cutlets (work in batches if needed) and cook 3-4 minutes per side until golden brown and internal temperature reaches 160-165°F (about 70-74°C). Transfer chicken to a plate and tent loosely with foil.

Vegetables & Sauce

Sauté aromatics

Reduce heat to medium. Add remaining 1 tablespoon olive oil and 1/2 tablespoon butter to the same skillet. Add sliced onion and cook 2-3 minutes until softened. Add minced garlic and cook 30-45 seconds until fragrant, stirring constantly to avoid burning.

Cook mushrooms & peppers

Add sliced mushrooms and red bell pepper to the skillet. Cook 5-6 minutes, stirring occasionally, until mushrooms have released moisture and peppers are tender-crisp and lightly browned. Season vegetables with remaining 1/2 tsp salt and remaining 1/2 tsp black pepper.

Build the sauce

Pour in 1 cup chicken broth to deglaze the pan, scraping up browned bits from the bottom. Bring to a simmer and let reduce for 2-3 minutes. Stir in 3 tablespoons ranch dressing until incorporated and smooth. If you prefer a thinner sauce, add an additional 1-2 tablespoons chicken broth. Taste and add 1 teaspoon lemon juice if desired for brightness.

Finish

Return the seared chicken and any accumulated juices to the skillet, nestling into the sauce and vegetables. Spoon sauce over the chicken and simmer gently for 3-4 minutes until chicken is fully cooked and sauce has slightly thickened. If using, sprinkle 2 tablespoons grated Parmesan over the chicken and let melt into the sauce.

To Serve

Transfer chicken, mushrooms and peppers to a serving platter or plate over cooked rice or mashed potatoes (about 2 cups prepared). Spoon extra ranch-mushroom sauce over the top and garnish with 2 tablespoons chopped fresh parsley. Serve immediately.