The traditional ratatouille recipe wants the vegetables to be brown in olive oil in a frying pan, each one separately. Do not peel them at all. All the vegetables are cooked with their skin to preserve a maximum of taste. Then, they will be mixed and will confit together in an earthenware casserole.
Several variants exist: replace the thyme with basil or add onions, according to taste. Making a homemade ratatouille is not complicated, it's just a bit long, but it's worth it.