Set the mackerel fillets with the skin side up on the parchment-lined baking sheet. Pour some olive oil over the fish and dredge with some salt and pepper to taste. Roast in the oven for 8-10 minutes. Crisp the fish and remove once cooked through. Reserve.
Set the stove to low. Heat the butter in a small skillet. Quarter the mushrooms and gently drop on the pan. Stir occasionally for about 10 minutes until the color turns golden. Top with the chopped parsley and enhance the flavor with a dash of salt and pepper.
Serve the fillet with the butter mushroom and an optional low carb salad.