Peel the eggplant and dice it into cubes. Mix with some salt and set it aside in a colander or strainer. Let it sit for about 30-45 minutes.
Chop onions, bell pepper, zucchini, and tomatoes to be close to the same size. Smash and mince garlic. Set aside until ready to use.
Pat the eggplant dry with a paper towel.
Preheat the oven to 400°.
Preheat a 12-inch cast iron skillet or another over-safe pan over medium-high heat.(If you don't have a pan that can go from the stove-top to the oven, you can always transfer the vegetables into a casserole dish for the baking step.)
When the skillet is preheated, add about 4 tablespoons of olive oil and eggplant. Stir to coat eggplant with oil and let it cook and caramelize until starts to soften and brown.
Take eggplant out of the skillet and set aside.
Add another tablespoon or two of olive oil the the pan and add onions, zucchini, and bell pepper. Sauté these vegetables until starts to soften and brown.
Make a well in the middle of the vegetables and add garlic oregano, and thyme sprigs. Let it sear until fragrant and add balsamic vinegar. Simmer for a few seconds and mix everything well.
Add tomatoes, salt, and pepper. You may need a little more olive oil.
Add eggplant back into the pan and mix everything until evenly incorporated.
Sauté for a few minutes and move the skillet over to the oven. Let it roast for about 20-25 minutes and take it out of the oven.
Serve ratatouille as a side dish, mixed with pasta, or on toasted baguette as an appetizer.