Preheat the oven to 400 degrees. Coat the bottom of your cast iron skillet with a teaspoon or two of olive oil; swirl to coat evenly.
Clean the mushrooms then slice the bigger ones in half or quarters, depending on the size. Place the mushrooms, shallot, garlic slivers in a bowl.
Drizzle the melted butter over the mixture then season with some fresh thyme leaves and sea salt & freshly cracked pepper, to taste. Toss to coat evenly.
Pour the mixture into the prepared cast iron skillet. Top with a sprig of fresh thyme.
Place into the oven and roast for 10-12 minutes making sure to stir them halfway through roasting. Serve immediately. Enjoy.