Finely mince the onion. Coat the bottom of a 10" skillet with olive oil, then transfer the onion to the skillet.
Add the crushed tomatoes + their juices to the skillet, followed by ½ teaspoon of salt, ½ teaspoon of garlic powder, a few cracks of pepper, and stir around. Place 4-5 leaves of basil in to the crushed tomatoes.
Using a sharp knife, thinly slice your eggplant, zucchini, yellow squash, and tomatoes into rings about ¼" thick.
Assemble the vegetables in the skillet. Alternating colors and trying to match like-sized slices with like sized-slices, places your veggie slices on edge around the outside of your skillet. Once the outer ring is complete, you can fill in the middle.
In a small bowl or ramekin, add 1 teaspoon of salt, ½ teaspoon garlic powder, and a few cracks of pepper. Then add ¼ cup of olive oil. Using a silicone brushed, mix the oil and spices together until combined, and brushing the mixture over the vegetables.
Cover the skillet with a lid or piece of aluminum foil and place it on your campfire grill grate, over medium low indirect heat, preferably over hot embers. Depending on your specific conditions, the cook time should take roughly an hour. Be sure to keep the fire going off to the side, so you can replenish the embers once they start to die out
The ratatouille is done when the veggies are cooked through and tender, but not mushy. Serve with pasta, rice, bread, or alongside grilled meat. Enjoy!