Season the chicken (if cooking): Pat chicken dry. Rub with olive oil, smoked paprika, garlic powder, onion powder, salt, and pepper.
Let sit 10 minutes while you prep the salad.
Cook the chicken: Grill over medium-high heat 5–7 minutes per side (breasts) or until juices run clear, or roast at 400°F (200°C) for 18–22 minutes. Rest 5 minutes, then slice or shred.
Make the dressing: In a small jar, combine olive oil, lemon juice, Dijon, garlic, salt, pepper, and red pepper flakes. Shake until emulsified.
Taste and adjust lemon or salt.
Prep the veggies: Chop romaine, add spinach, slice cucumbers and onion, halve tomatoes, and chop herbs. Slice or cube the avocado just before assembling.
Assemble the bowl: Add greens to a large bowl. Top with chicken, avocado, cucumber, onion, tomatoes (if using), herbs, and seeds or nuts.
Sprinkle with feta if desired.
Dress and serve: Drizzle with dressing and toss gently to coat. Finish with an extra squeeze of lemon and a pinch of salt if needed. Serve immediately.