1. To roast the vegetables: Preheat the oven to 400 degrees F. Place the eggplant, zucchini, yellow squash, red pepper, onion, and garlic on a large baking sheet. Drizzle vegetables with olive oil and gently toss. Season with salt and pepper. Place the pan in the oven and roast for about 30-35 minutes, stirring vegetables occasionally. Remove vegetables from the oven and set aside.
2. In a medium saucepan, bring three cups of water to a boil. Add a generous pinch of salt, stir in the Israeli couscous. Reduce heat to a simmer. Simmer until couscous is just tender, with a slight bite, about 10 to 13 minutes. Drain into a fine mesh strainer and rinse with cold water. Set aside.
3. In a large bowl, combine couscous and roasted vegetables. Stir in the basil and feta cheese.
4. To make the dressing: In a small bowl, combine the vinegar, mustard, and garlic. Add the oil in a slow steady stream, whisking constantly. Season with salt and pepper to taste. Pour dressing over couscous and vegetables and stir until well combined. Serve at room temperature or chilled.