Season thighs with salt, pepper, and thyme. Sear skin-side down in olive oil over medium-high heat until golden, 6–8 minutes; flip and brown 3 minutes.
Whisk honey, soy sauce, garlic, and vinegar. Pour over chicken, reduce heat to medium-low, cover, and simmer 20–25 minutes until internal temp reaches 165°F.
Meanwhile, bring broth to a boil, add quinoa, cover, and simmer 15 minutes until fluffy. Stir in spinach off heat so it wilts.
Remove chicken, spoon sauce over, and serve thighs atop quinoa. Garnish if desired.