Heat the oven to 425ºF. Heat the oil in a non-stick skillet and add the garlic. Fry briefly, stirring just to soften. Add in the spinach and toss until wilted. Spoon the spinach into the bottom of two gratin pans.
Sit the cod on top of the spinach then top the cod with the feta cheese sprinkle and the diced tomatoes.
Sprinkle with a few fresh thyme leaves and bake for about 12 minutes or until the fish flakes easily. Bring to the table in the gratin dishes.