Cut chicken into even 1-inch / 2.5 cm pieces.
Whisk olive oil, lemon juice, optional yogurt, garlic, tomato paste, paprika, oregano or thyme, cumin, salt, black pepper and red pepper flakes in a bowl.
Add chicken and coat well. Cover and refrigerate for at least 2 hours, ideally 6 to 12 hours.
Soak wooden skewers in water for 30 minutes if using.
Thread chicken onto skewers with peppers and onions, leaving small gaps between pieces.
Preheat grill or grill pan to medium-high heat and lightly oil the grates.
Grill for 10 to 12 minutes, turning occasionally, until lightly charred and cooked through.
Check that chicken reaches 165°F / 74°C internally.
Rest for 3 to 5 minutes and serve with lemon, herbs, pilaf and yogurt sides.