Heat 1 tablespoon of olive oil in a large ovenproof Dutch oven over medium heat.

Add the onion and cook, stirring often, until softened, about 6 minutes.

Add the remaining 1 1/2 tablespoons of olive oil, eggplant, and 1/2 teaspoon of salt and cook, stirring often, for 3 minutes.

Add the bell pepper and garlic and cook, stirring often, until the eggplant begins to soften, 6 to 7 minutes.

Stir in the tomatoes and zucchini.

Cover partially, reduce heat to low, and simmer, stirring often to prevent sticking, for 10 minutes.

Add the parsley, tomato paste, vinegar, basil, the remaining 1/2 teaspoon of salt, and freshly ground pepper to taste.

Simmer, uncovered, for 3 minutes.

Remove from heat, smooth the top with a spoon, and cool for 1 hour.

Meanwhile, make the soft polenta according to your recipe.

As soon as the polenta is quite thick but still pourable, immediately scrape the warm polenta over the ratatouille, spreading evenly with a spoon.

Let cool for 30 minutes.

Preheat oven to 375F (190C).

Cover the Dutch oven with a lid or foil and bake until heated through, 35 to 40 minutes.

Remove from oven and let stand, covered, for 10 minutes before serving.