Preheat a slow cooker to low. Place the oil, garlic, thyme, rosemary, basil, peppercorns, and halibut fillets in the slow cooker insert. Keep the fish in a single layer; do not stack the fillets or they stick together.
Cover and cook on low for three hours, until the fish is firm and cooked through.
Gently remove the fish with a slotted spatula. Strain the oil and store, covered, in the refrigerator. The oil may be used up to three times for poaching fish.
Garnish with chopped herbs and serve with your choice of accompaniment.