Fire up the grill to 375 degrees F, or medium-hot heat
Oil the grill liberally with olive oil.
Be careful to remove all the bones from the fish.
Season the fish with half the salt, pepper, cumin, and oregano on both sides.
Cook 4-5 minutes on each side, depending on the thickness of the fish.
In the meantime, in a small bowl, add mayonnaise (or Greek Yogurt, if using), lemon juice, chipotle and remaining salt, pepper, cumin, oregano.
Mix together and set aside.
Heat up corn tortillas and top with grilled fish, chipotle mayo sauce, spinach, and tomatoes.