Season halibut fillets evenly with salt, pepper and garlic powder.
Melt half of the ghee (or coconut oil) in a large skillet over medium-high heat; add halibut and cook for 3 to 5 minutes per side or until fish flakes easily when tested with a fork; remove from skillet and keep warm.
Add remaining ghee (or coconut oil) to the skillet along with the mushrooms; sauté for 1 minute. Add garlic and sauté for 1 minute. Add wine and vinegar then reduce heat and simmer for 5 to 10 minutes or until sauce is slightly reduced.
Serve over halibut fillets.