In a bowl, whisk olive oil, lemon juice, garlic, oregano, thyme, parsley, paprika, salt, pepper, and Dijon mustard until combined.
Add chicken and mix well until fully coated.
Cover and marinate in the refrigerator for at least 1 hour.
Preheat grill to medium-high heat (or oven to 425°F / 220°C).
Thread chicken, peppers, and onion onto skewers.
Grill for 10–12 minutes, turning occasionally, until chicken reaches 165°F (74°C).
OR bake for 18–22 minutes, flipping halfway.
Remove from heat and let rest for 5 minutes.
Garnish with parsley, lemon wedges, or feta if desired.
Serve warm.