1. Make the chicken: Preheat the oven to 425˚F (220˚C).
2. Pat the chicken thigh dry and season with olive oil, salt, and pepper. In a cast-iron skillet over high heat, sear the chicken thigh, skin-side down, until the skin is crispy, about 10 minutes. Transfer the skillet to the oven and roast for 15 minutes, until the chicken is cooked through.
3. Cook the gnocchi according to the package instructions, then drain.
4. Meanwhile, in a small pan over medium heat, sauté the onion and tomato in a drizzle of olive oil until the onion is translucent and fragrant and the tomato is tender, about five minutes. Add the spinach and continue cooking until wilted. Add the canned tomatoes and continue cooking until the sauce is thickened and warm, about 5 minutes. Season with salt, pepper, and red pepper flakes. Add the cooked gnocchi to the sauce and toss to coat.
5. Plate the gnocchi with the whole piece of crispy chicken.