Preheat broiler. Make vinaigrette: Whisk vinegar, mustard, salt, pepper, and 2 tsp. harissa in a bowl. Gradually whisk in olive oil. Taste; add more harissa if you'd like a hotter dressing. Set aside.
Make salad: Put peppers on a baking sheet and broil, turning periodically, until charred on all sides, about 20 minutes. Place peppers in a paper bag about 15 minutes to allow trapped steam to loosen their skins. Remove charred skins, stems, and seeds; cut peppers into 1/2-in. pieces.
Meanwhile, heat olive oil in a large nonstick frying pan over medium heat. Add eggplant and cook, stirring often, until softened and beginning to color, about 12 minutes. Add chickpeas, 2 tbsp. water, and 3/4 tsp. each salt and pepper; cook, stirring, to blend flavors, about 3 minutes. Stir in chopped peppers. Remove from heat and let stand until vegetables are warm but not hot.
In a large bowl, toss spinach, mint, and red onion with half the vinaigrette. Add chickpea mixture, drizzle with additional dressing, and toss again to coat. Season to taste with more salt and pepper.
*Buy harissa, a north African chile-and-spice paste, in the international section of many supermarkets. Heat level varies greatly from brand to brand, so try a little before using.
Note: Nutritional analysis is per serving.