Pre-heat grill. Combine the garlic, salt, lime juice, first portion of olive oil, sugar, and chipotle chili pepper in a mortar and pestle or a food processor. Process to make a smooth paste.
Place the fish on a tray and spoon the chili paste evenly on top of each piece of fish. Wrap each piece of fish in plastic wrap and marinade for about 30 minutes in a fridge.
Unwrap the fish and place skin side down on the grill; if you have a metal lid for the grill, use this to cover the fish. Each piece of fish should cook through in about 8-10 minutes, until an internal temperature of 145°F has been reached. Sauté the spinach in the second portion of olive oil and serve with the fish.
Notes
The fish may also be wrapped in a flexible cedar plank and cooked in the oven.