In a small bowl, combine the olive oil, lemon juice, lemon zest, garlic, oregano, thyme, capers, Dijon mustard, salt, pepper. Whisk until slightly emulsified.
Place chicken (or fish) in a shallow dish or resealable bag. Pour over the marinade and coat evenly. For chicken, marinate at least 30 minutes and up to 12 hours. For fish, marinate for 20 to 30 minutes.
Cooking Times
Grill: Medium-high heat until lightly charred and cooked through. About 5 to 7 minutes per side for chicken breasts and 3 to 4 minutes for fish.
Bake: 400°F oven for 20 to 25 minutes for chicken and 10 to 12 minutes for fish.
Pan-sear: Medium high heat in a heavy pan for about 5 to 7 minutes per side for chicken breasts and 3 to 4 minutes for fish.
Notes
Make Ahead - you can save this marinade in the fridge for up to 4 days in an airtight container.