Preheat the oven to 400F.
Slice the zucchini, squash, and eggplant into thin rounds and add them to a large bowl. The slices should be thin enough to slightly see through them, about 1/8 inch.
Sprinkle the sliced vegetables with the salt and toss with your hands to evenly coat them in the salt.
Let the vegetables sit at room temperature for 10 minutes to draw out extra moisture from them.
While the vegetables are sitting, heat up 2 tbsp of olive oil in a cast-iron skillet over medium-high heat.
Add the onion, bell peppers, and garlic. Sautee for 10 minutes until vegetables are tender. Add the crushed tomatoes and simmer for another 10 minutes.
Remove the tomato mixture from heat and add to a blender. Blend until you get a smooth sauce.
Spoon 2 tablespoons of tomato sauce on to the bottom of the skillet.
Squeeze out any extra moisture from the vegetables and layer the slices in the skillet, repeating between zucchini, squash, and eggplant. Spoon 4 tablespoons of sauce in between each layer of vegetables.
After the layers are completed, top the ratatouille with a few spoonfuls of sauce, and drizzle with 2 tablespoons of olive oil.
Bake the ratatouille uncovered for 25 minutes until bubbling and the top vegetables are wilted.