To make ratatouille in a slow cooker, first line the cooker with a slow-cooker liner, and spray it with a nonstick cooking spray. Julienne 1 large onion, and layer half of it on the bottom of the slow cooker. Peel 1 large whole eggplant, and slice it into 1/4-inch pieces. Layer half of the eggplant on top of the onion layer.
Peel 2 whole zucchinis and 1 whole yellow squash, and slice them into 1/4-inch pieces as well. Reserve half of the squash, and place the other half on top of the eggplant. Mince 3 cloves of garlic, and dice 1 whole tomato. Spread half of the garlic and half of the tomato over the squash layer.
In a separate bowl, combine 8 ounces of Italian-seasoned tomato sauce with 1 teaspoon each of dried basil, dried oregano, granulated sugar and salt. Mix in 1/2 teaspoon of freshly ground black pepper and 1 tablespoon of dried parsley. Pour the sauce and herb mixture over the vegetables in the slow cooker. Add the reserved vegetables in layers in the same order as before, topping the layers off with the remaining tomato and garlic.
Drizzle 1/4 cup of olive oil over all of the ingredients. Cook the ratatouille on the low heat setting for seven to nine hours. If you desire, serve the ratatouille over cooked rice.