Preheat your oven to 400°F and line a small baking sheet with parchment paper.
Clean and slice the cremini mushrooms, then toss them on the baking sheet with the olive oil, half of the sea salt, pepper, and the fresh thyme.
Roast the mushrooms for 18-20 minutes until they are tender and have developed a golden-brown edge.
While the mushrooms roast, season the chicken breast with the remaining salt and pepper.
Heat a non-stick skillet over medium-high heat and sear the chicken for about 6-7 minutes per side until the internal temperature reaches 165°F.
Remove the chicken from the skillet and let it rest for 3 minutes before slicing.
In the same skillet, lower the heat to medium and melt the ghee. Add the minced garlic and sauté for 1 minute until fragrant.
Stir the roasted mushrooms and baby spinach into the garlic ghee, tossing for 1-2 minutes until the spinach is just wilted.
Finish with a squeeze of fresh lemon juice and serve the garlic butter mushrooms and greens alongside the sliced chicken.