Rinse the chicken livers under cold water and pat them thoroughly dry with paper towels to ensure a proper sear.
Melt half of the ghee in a large skillet over medium heat, then add the thinly sliced yellow onion.
Sauté the onions for 10-12 minutes, stirring occasionally, until they are soft and deeply caramelized.
Push the onions to the outer edges of the pan, add the remaining ghee to the center, and increase the heat to medium-high.
Season the chicken livers with sea salt and black pepper, then place them in the skillet in a single layer.
Sear the livers for 2-3 minutes per side until browned on the outside but still slightly pink and tender in the center.
Drizzle the balsamic vinegar over the pan and sprinkle with fresh thyme, tossing everything together to glaze the livers and onions.
Add the baby spinach to the pan and toss for 1 minute until just wilted, then serve immediately.