Mix together orange zest, orange juice, 1 tbsp olive oil and fresh thyme in a shallow baking pan. Place chicken breasts in the dish and turn to coat.
Mix brown sugar, cumin, coriander, Mexican oregano and salt on a large dinner plate. Spread out the spices. Add each chicken breast, one by one, and coat both sides with the seasoning. Put chicken back into the baking dish. Cover and marinate in the fridge for 30 minutes.
Pre-heat oven to 350°F. Bake chicken for 45 minutes, or until cooked. Remove cover during last 10 minutes of baking time.
In a large skillet heat 2 tbsp of olive oil over medium heat. Saute garlic for 2 minutes. Add in spinach and toss. Stir until spinach is wilted and coated in the garlic and oil.
To serve, place chicken on a bed of wilted spinach. Serve hot.