(30 mins to 12 hours ahead of time): Whisk marinade ingredients together in a small bowl first. Place fish steaks in a sealed plastic bag or a glass pyrex dish and pour the marinade over the top. Place sealed in fridge until you're ready to cook.
If using a grill: spray grates with an olive oil spray and heat grill to medium-high heat. Let warm up. Turn heat up to high heat and place ahi steaks on the grill. Cook for 1-3 minutes and flip. Cook for 1-2 minutes more (or less, if you enjoy a little pink in the middle).
If using the stovetop: Heat olive oil in a cast iron skillet over medium-high heat, and let pan warm up. Turn heat up to high, and place ahi on pan. Cook for 1-3 minutes and flip. Cook for 1-2 minutes more (or less, if you enjoy a little pink in the middle).
For either method: remove fish to a cutting board once done and slice into pieces. Keep heat source on and quickly warm tortillas by adding them to the pan/grill for 1-2 minutes per side.
Assemble tacos: assemble fish amongst 8 tortillas. Add mayo, salsa, avocado and anything else you'd like to each taco. Serve hot with extra lime wedges.