Preheat your oven to 400°F (200°C) and line a small baking sheet with parchment paper.
Clean the cremini mushrooms and cut them into halves or quarters so they are uniform in size.
In a small bowl, melt the ghee and whisk in the minced garlic, chopped fresh thyme, half of the sea salt, and half of the black pepper.
Toss the mushrooms in the garlic-ghee mixture until thoroughly coated, then spread them in a single layer on the prepared baking sheet.
Roast the mushrooms for 18-20 minutes, tossing halfway through, until they are golden brown and tender.
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 5-7 minutes per side until the internal temperature reaches 165°F.
Let the chicken rest for 3 minutes before slicing into strips.
Place the fresh baby spinach on a plate, top with the roasted garlic mushrooms and sliced chicken, and finish with a bright squeeze of lemon juice.