Prepare a 9 x 13-inch baking dish or a large bowl for the marinade.
In the dish or bowl, whisk together 1/2 cup extra virgin olive oil, the juice of 2 lemons (about 1/4 cup), the lemon zest, minced garlic, chopped parsley, dried oregano, salt, dried thyme, ground black pepper, and crushed red pepper if using.
Add the 3–4 pounds of boneless chicken breasts or thighs to the marinade and turn each piece to coat thoroughly.
Cover the dish with plastic wrap and refrigerate for at least 30 minutes and up to 4 hours.
To grill: preheat the grill to medium (about 350–400°F). Grill the chicken 5 minutes per side or until cooked through.
To bake: preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper and arrange the chicken pieces so they do not touch. Bake 15–20 minutes, until the thickest piece is cooked through.