Add the olive oil to a large skillet set over medium heat. When the oil is hot, add in the onion and cook for about 5 minutes, until tender. Add in the garlic and cook for 30 seconds, until fragrant. Add in the zucchini, squash and bell pepper. Cook for about 3 to 5 minutes, until the veggies have softened. Add in the diced tomatoes, oregano and sugar. Continue to cook, stirring occasionally, for about 5 minutes, until the veggies are all tender. Remove from the heat and stir in the balsamic vinegar, salt and red pepper flakes.
For the polenta stacks:
While the ratatouille simmers, set a grill pan over medium high heat and allow it to heat up for several minutes. It is hot enough when a drop of water sizzles on the pan. While the pan is heating, brush the eggplant and polenta slices all over with olive oil, and sprinkle just the eggplant with salt and pepper.
Working in batches, place the eggplant and polenta slices on the pan in a single layer and cook until tender and charred, flipping once halfway through. The eggplant will take about 3 to 5 minutes per side, and the polenta will take about 3 minutes per side.
To assemble, divide the ratatouille mixture among 4 shallow bowls. Stack a few slices of the grilled polenta and eggplant in alternating layers on top, then sprinkle with fresh basil. Serve immediately.
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