This vegetable-packed dinner features the heat-loving produce that thrives before the cooler nights of fall set in.
Preheat oven to 375°F. Bring water to a boil in a small saucepan over medium-high heat. Add farro and reduce heat to maintain a simmer. Cover and cook until tender, about 25 minutes. Remove from heat, drain and transfer to a medium bowl; let cool to room temperature.
Meanwhile, slice tops off bell peppers and remove the seeds. Chop the tops (discard the stems) and set aside. Place the peppers in an 8-inch-square microwave-safe baking dish. (If they don't stand upright, trim a thin slice off the bottom, without cutting through.) Add 1 tablespoon water to the dish and microwave on High for 5 minutes. Season with 1/4 teaspoon salt and 1/8 teaspoon pepper. Cover with foil and set aside.
Heat 2 tablespoons oil in a large skillet over medium-high heat. Add eggplant and cook, stirring frequently, until lightly browned in spots, 5 to 7 minutes. Transfer to a large bowl.
Reduce heat to medium and add 1 teaspoon oil to the pan. Add zucchini and the reserved chopped bell pepper and cook until browned in spots, 5 to 6 minutes. Transfer to the bowl.
Add tomato, onion, thyme and the remaining 1/2 teaspoon salt and 1/8 teaspoon pepper to the pan; cook, scraping the bottom of the pan, until the onion is tender and the tomato has broken down, about 4 minutes. Add garlic and marjoram and cook for 1 minute more. Transfer to the bowl.
Stir beans, 1/2 cup cheese and 1/4 cup basil into the vegetable mixture. Spoon about 1 cup into each pepper, mounding as needed. Spoon any remaining filling around the peppers. Cover the pan with foil. Bake until the peppers are tender, about 30 minutes. Remove the foil and sprinkle them with the remaining 1/2 cup cheese. Bake until the cheese is melted, 10 to 15 minutes more.
Meanwhile, whisk vinegar and mustard in a large bowl. Slowly drizzle in the remaining 4 tablespoons oil while continuing to whisk. Add the remaining 1/4 cup basil, arugula and the reserved farro; toss to combine. Garnish the peppers with more basil, if desired, and serve with the farro salad.
Refrigerate cooked farro, bell peppers and ratatouille (Steps 1-5) separately for up to 2 days.