In a non-reactive vessel, combine 2 tablespoons olive oil, lemon juice, lemon zest, Dijon mustard, 3 minced garlic cloves, 1 teaspoon dried oregano, 1 teaspoon dried thyme, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Whisk vigorously until emulsified.
Submerge the bone-in, skin-on chicken thighs into the marinade mixture to coat thoroughly. Allow to marinate at ambient temperature for 15 minutes.
Place a heavy-bottomed skillet or deep pan over medium-high heat and add 1 tablespoon olive oil.
Extract the chicken thighs from the marinade, reserving leftover liquid. Place chicken skin-side down in the hot skillet and sear for 5 to 7 minutes until the skin renders fat and turns deeply golden brown.
Flip chicken thighs and sear the reverse side for 3 minutes. Remove chicken from skillet and set aside on a plate.
Reduce heat to medium. Add the diced yellow onion to the rendered pan juices and sauté for 3 minutes until translucent.
Incorporate 2 minced garlic cloves, 1 teaspoon dried oregano, and 1 cup long grain white rice. Stir continuously for 1 to 2 minutes to toast the grains and infuse aromatics.
Deglaze the skillet by pouring in 2 cups chicken stock and the reserved marinade liquid, scraping up all fond attached to the bottom of the pan.
Stir in 1/2 teaspoon salt, 1/4 teaspoon black pepper, and 2 cups roughly chopped baby spinach, stirring until the greens begin to collapse into the broth.
Nestle seared chicken thighs back into the pan, skin-side up, placing them on top of the rice and liquid base.
Bring the mixture to a rapid boil, then immediately turn heat to low and cover with a tight-fitting lid.
Simmer undisturbed for 20 to 25 minutes until liquid is fully absorbed, rice is tender, and chicken reaches an internal core temperature of 165°F (74°C).
Remove from heat and let rest covered for 5 minutes. Garnish with chopped parsley and additional lemon zest or slices prior to serving.