Start by placing in the blender the garlic clove, 1/2 cilantro, 1/4 onion, and 1/4 teaspoon salt. Blend until liquified.
Place the fillets in a container and sprinkle salt and pepper on top. Cover with the marinade.
Cut a piece of foil and place the piece of fish in the middle. Rinse with some marinade, add slices of onion, tomato, and jalapeno to taste, and add some salt on top as well as a couple of branches of cilantro. Cover with another piece of aluminum and fold foil twice on the edges so the marinade can stay inside while cooking or use foil packs if you have.
Place water in the steamer and allow to boil. Add the tilapia inside the steamer, cover, and allow to cook 15-20 minutes.