Add chicken thighs, harissa paste, salt, pepper, onions, and olive oil to plastic bag or large bowl and toss to marinate, covered, in the refrigerator, for at least 30 minutes up to overnight.
Preheat oven to 350°F and arrange rack to center position.
Add chicken thighs to pan with onions.
Cook 20 minutes. Flip chicken thighs and stir onions.
Stir in stock and spinach to pan with chicken and onions.
Bake additional 20 minutes, until chicken has reached 165 degrees internal temperature with a digital thermometer.
If you'd like a bit of extra browning, you can broil chicken or pop in a pan to sear.
Serve with rice, pita, vegetables, or potatoes.
Enjoy!