Pat the cod fillet dry with paper towels and season both sides with salt and pepper. Slice the zucchini into half-rounds and mince the garlic.
Heat half the olive oil in a skillet and sauté the zucchini until golden brown, about five minutes. Add the garlic, cook briefly and then stir in the spinach until wilted. Season to taste and keep warm.
Heat the remaining oil in another pan and pan-fry the cod for three to four minutes per side until the fish is opaque and flakes easily. Place the fish on the vegetables and squeeze some lemon juice over just before serving.