Pat the chicken thighs dry with paper towels to help them brown evenly. In a small bowl, combine the rosemary, smoked paprika, salt, and pepper, then rub this seasoning mixture generously over both sides of each chicken thigh. While the chicken sits for a few minutes, finely dice the onion, slice the mushrooms, and mince the garlic—having everything prepped will allow the cooking to move quickly without interruption.

Heat the olive oil in a large skillet over medium-high heat until shimmering. Working in batches if needed to avoid crowding, sear the seasoned chicken thighs for 2 minutes per side until they develop a golden-brown crust. This browning creates deep, savory flavors that will enhance the entire dish. Transfer the seared chicken to a clean plate and set aside.

In the same skillet (don't clean it—those browned bits add flavor), sauté the diced onion and sliced mushrooms for 4-5 minutes until the mushrooms begin to soften and release their moisture. Add the minced garlic and cook for 1 minute until fragrant. Stir in the dry quinoa and toast it for about 1 minute, stirring constantly—this toasting step brings out the grain's nutty flavor and helps it absorb the broth more evenly.

Pour in the chicken broth, stirring to combine with the quinoa and vegetables. Bring the mixture to a boil, then reduce the heat to low, cover the skillet with a lid, and simmer for 10 minutes. The quinoa will absorb most of the liquid during this time and begin to cook through.

After the 10-minute simmer, carefully nestle the seared chicken thighs into the quinoa mixture, partially submerging them. Cover the skillet again and simmer for another 20 minutes until the chicken is cooked through and tender (it should reach an internal temperature of 165°F). I like to add the red pepper flakes to the quinoa at this point so their heat gently infuses the entire dish without overpowering it.

Remove the chicken to a serving plate. While the quinoa is still hot, stir in the tightly packed spinach and fresh lemon juice, mixing until the spinach wilts completely into the grains (this takes just 1-2 minutes). The lemon juice brightens the dish and cuts through the richness of the chicken. Taste the quinoa mixture and adjust seasoning with additional salt and black pepper as needed.

Divide the warm quinoa and vegetable mixture among serving bowls or plates, then top each portion with a chicken thigh. The chicken should rest on top of the quinoa, making for an attractive presentation where each element is visible.