Marinate the chicken: In a bowl, whisk olive oil, lemon juice, garlic, smoked paprika, oregano, salt, and pepper. Coat chicken and let sit 15–30 minutes (or up to 8 hours in the fridge).
Cook the quinoa: Rinse quinoa well.
Add to a pot with broth and a pinch of salt. Bring to a boil, reduce to low, cover, and cook 15 minutes. Turn off heat and let steam 5 minutes.
Fluff with a fork.
Prep the grill or pan: Heat a grill or grill pan to medium-high. Lightly oil grates (or the pan).
Grill the chicken: Cook 5–7 minutes per side until internal temp reaches 165°F (74°C). Rest 5 minutes, then slice.
Make the vinaigrette: Whisk lemon juice, vinegar, olive oil, Dijon, honey, salt, and pepper until smooth.
Assemble the bowls: Add a bed of greens to each bowl.
Spoon in quinoa, top with sliced chicken, tomatoes, cucumber, bell pepper, onion, and herbs. Add avocado if using.
Dress and serve: Drizzle with vinaigrette right before eating. Finish with a pinch of salt, pepper, and a squeeze of lemon if you like.