If your chicken breasts are particularly thick, you can lightly pound them to an even thickness (about 3/4 inch or 2cm) for more uniform cooking. Alternatively, you can slice thicker breasts in half horizontally to create thinner cutlets. Pat the chicken dry with paper towels.
Combine Marinade Ingredients: In a medium-sized non-reactive bowl (glass or ceramic is ideal), whisk together the extra virgin olive oil, fresh lemon juice, red wine vinegar, and Dijon mustard until lightly emulsified.
Add Aromatics: Stir in the minced garlic, dried oregano, chopped fresh rosemary, sea salt, black pepper, and the optional pinch of red pepper flakes. Mix thoroughly to ensure all seasonings are well distributed.
Marinate the Chicken: Add the chicken breasts to the bowl with the marinade. Turn each piece to ensure it's fully coated. Cover the bowl tightly with plastic wrap or transfer the chicken and marinade to a resealable plastic bag, pressing out any excess air. Refrigerate for at least 1.5 hours, or up to 3 hours for deeper flavor. Do not exceed 4 hours, as the acidity can start to negatively affect the chicken's texture.