Step 1: Prepare and Season the Chicken
Pat the chicken breasts completely dry with paper towels. This is crucial for a good sear. In a small bowl, combine 1 tbsp olive oil, the minced garlic, lemon zest, thyme, rosemary, salt, pepper, onion powder, and red pepper flakes. Rub this mixture all over the chicken breasts. Let them sit at room temperature for 15-20 minutes while you prep vegetables.

Step 2: Sear the Chicken
Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a large, heavy-bottomed skillet (cast iron or stainless steel) over medium-high heat. Once the butter is melted and foaming, add the chicken breasts. Sear without moving for 5-7 minutes, until a deep golden-brown crust forms. Flip and cook for another 5-7 minutes, or until cooked through (internal temperature of 165°F / 74°C). Transfer chicken to a clean plate and tent loosely with foil.

Step 3: Sauté the Vegetables
If using hardy vegetables like potatoes or broccoli, add them to the skillet after removing the chicken, with a drizzle of oil if needed, and cook for 5-7 minutes to begin browning. For quicker-cooking veggies like asparagus, zucchini, or tomatoes, you’ll add them later. Add the sliced onion to the skillet and cook for 3-4 minutes until starting to soften.

Step 4: Build the Aromatic Base
Push vegetables to the side. Add the remaining 1 tablespoon of butter to the center of the skillet. Add the 3 cloves of minced garlic and cook for 30-60 seconds until fragrant, being careful not to burn it. If using flour for thickening, sprinkle it over the garlic and butter and cook for 1 minute, stirring.

Step 5: Deglaze and Create the Sauce
Pour in the white wine (or chicken broth) to deglaze the pan. Use a wooden spoon to scrape up all the flavorful browned bits (fond) from the bottom. Let it simmer for 2-3 minutes until reduced by half. Stir in the chicken broth, lemon juice, and Dijon mustard (if using). Bring to a simmer.

Step 6: Finish Cooking Vegetables and Thicken Sauce
Add any remaining quick-cook vegetables (asparagus, zucchini, tomatoes) to the simmering sauce. Let everything cook for 4-5 minutes, until the vegetables are tender-crisp and the sauce has reduced slightly. If using heavy cream, stir it in now and heat through. For a thicker sauce without cream, let it simmer a few minutes longer.

Step 7: Return Chicken and Combine
Return the seared chicken breasts and any accumulated juices to the skillet, nestling them into the sauce and vegetables. Spoon some of the sauce over the top. Let everything heat through for 2-3 minutes.

Step 8: Garnish and Serve
Remove from heat. Taste the sauce and adjust seasoning with more salt, pepper, or a squeeze of lemon if needed. Garnish generously with fresh chopped parsley and lemon slices or wedges. Serve immediately directly from the skillet.