Cut an 18. 0-inch (45. 7cm) piece of heavy-duty aluminum foil, place the halibut fillet directly in the middle of the sheet, and season it evenly on both sides with the salt and black pepper.
Whisk together the dried dill, fresh lemon juice, and avocado oil in a small bowl until the emulsion is well combined.
Drizzle the lemon and oil mixture over the halibut fillet, then carefully fold and crimp the edges of the foil to create a sealed pouch, ensuring no steam can escape.
Place the foil-wrapped fish into a 6. 0-quart (5. 7L) slow cooker, ensuring it sits flat on the bottom.
Plug in the slow cooker, secure the lid tightly, and cook for 1. 5 hours to 2. 0 hours on the high heat setting until the internal temperature reaches 130-135°F (54-57°C).
When the timer is up, carefully open the crimped foil edges (watch for hot steam!) and check the fish with a fork; it should be perfectly tender and flaky.
Check the Temperature Early: Halibut is a very lean fish and can go from perfectly flaky to overly firm in a matter of minutes. Start checking the internal temperature at the 1 hour and 15-minute mark. You are aiming for an internal temperature of 130°F to 135°F (54-57°C).