This can be executed on an indoor grill pan, but it will take forever. If that’s all you have at your disposal, save yourself the trouble, and just make regular ratatouille! Additional instructions on grilling veggies can be found in the July/August 2011 issue of Cooks Illustrated.
In a shallow dish, place half of the olive oil. Brush the eggplant, squash, zucchini, and peppers with oil on both sides and season with salt and pepper. String the tomatoes onto skewers and brush with oil. Arrange the veggies on the grill.
Grill on both sides until nice char marks form and each vegetable becomes tender (about 10 minutes for eggplant, 8 for zucchini/squash, 8 for peppers, 5 for tomatoes, 10 for onions).
Remove the veggies to a cutting board and roughly chop. Combine the basil, balsamic, and remaining oil in a mixing bowl. Add the chopped veggies and toss to combine. Taste for seasoning and add more salt as necessary.
Enjoy alongside grilled meat or fish, or as a topping for pasta, hot or at room temperature.