Cut zucchini in half length wise. Slice onions in thin half moons. Mince garlic and set aside.
Add 2 tablespoons of olive oil to a shallow dish and season with a pinch of salt. Add zucchini and give it a stir making sure that the zucchini slices are completely coated with olive oil.
Clean grill and place cast iron skillet on top of the grill on medium flame. Close lid and allow the cast iron to warm up for two minutes. Add olive oil and onion slices. Close lid and sauté onions for about 5 minutes…