Preheat your oven to 400°F and line a baking sheet with parchment paper.
Clean and slice the cremini mushrooms into thick pieces and finely mince the garlic cloves.
In a medium bowl, toss the sliced mushrooms with half of the melted ghee, half of the minced garlic, fresh thyme, and a pinch of sea salt and black pepper.
Spread the mushrooms in a single layer on the prepared baking sheet and roast for 15-20 minutes until they are golden brown and tender.
While the mushrooms roast, season the chicken breast on both sides with the remaining sea salt and black pepper.
Heat a cast-iron skillet over medium-high heat and add the remaining ghee to the pan.
Place the chicken in the skillet and sear for 6-8 minutes per side until the exterior is crispy and the internal temperature reaches 165°F.
During the last 2 minutes of cooking the chicken, add the remaining minced garlic and the baby spinach to the skillet, sautéing until the spinach is just wilted.
Plate the chicken over the bed of wilted spinach, top generously with the roasted garlic mushrooms, and finish with a fresh squeeze of lemon juice.