Thinly slice the yellow onion and the cremini mushrooms.
Heat a heavy cast-iron skillet over medium-low heat and add half of the ghee.
Add the sliced onions and mushrooms to the skillet, cooking slowly for 12-15 minutes until the onions are golden brown and caramelized.
Remove the onion and mushroom mixture from the pan and set aside on a plate.
Pat the ribeye steak dry with paper towels and season both sides generously with sea salt and black pepper.
Turn the skillet heat up to medium-high and add the remaining ghee.
Once the pan is shimmering, sear the steak for 3-4 minutes per side for a medium-rare finish.
In the final minute of searing, add the fresh thyme to the pan and spoon the hot ghee over the steak to baste it.
Transfer the steak to a cutting board and allow it to rest for 5 minutes to keep the juices intact.
While the steak rests, toss the baby spinach into the hot skillet for 30 seconds until just wilted.
Slice the steak against the grain and serve topped with the warm caramelized onions and mushrooms alongside the wilted spinach.