Prepare the Eggplant: Preheat your oven to 400°F (200°C). Place the eggplant chunks in a colander, sprinkle with 1/4 teaspoon of the salt, and toss to coat. Let stand for 20 minutes to draw out excess moisture. Pat the eggplant thoroughly dry with a paper towel, then transfer to a baking sheet. Drizzle with 1 tablespoon of olive oil, toss to coat, and roast for 20-25 minutes, flipping halfway, until golden brown and tender.
Build the Aromatic Base: While the eggplant roasts, heat the remaining 1 tablespoon of olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add the onion and sauté for 4-5 minutes until softened. Add the red bell pepper and celery and cook for another 5-7 minutes until tender.
Bloom the Spices: Add the sliced garlic and cook for 1 minute until fragrant. Stir in the ras el hanout, cumin, coriander, and cinnamon, and toast for 30 seconds, stirring constantly.
Assemble the Tagine: Pour in the diced tomatoes with their juices, the chickpeas, chopped Medjool dates, 1 cup of vegetable broth, the remaining 1/4 teaspoon of salt, and the black pepper. Bring the mixture to a simmer.
Simmer and Unify: Gently stir in the roasted eggplant and half of the chopped parsley. Reduce the heat to low, cover, and let it simmer for at least 15 minutes for the flavors to meld.
Cook the Couscous: While the tagine simmers, bring the 1.5 cups of vegetable broth to a boil in a medium saucepan. Remove from heat, stir in the whole wheat couscous, and immediately cover tightly. Let stand for 10 minutes, then fluff with a fork.
Prepare the Toppings: In a small bowl, whisk together the tahini and 2 tablespoons of lemon juice. The mixture will thicken. Add cold water, 1 tablespoon at a time, whisking until you reach a smooth, pourable, creamy consistency.
Finish and Serve: Remove the tagine from the heat. Stir in the juice of 1/2 lemon and the remaining fresh parsley. To serve, spoon the couscous into shallow bowls and ladle the tagine over top. Drizzle generously with the lemony tahini sauce and sprinkle with toasted slivered almonds.