Position 2 racks in the upper and lower thirds of the oven and preheat to 450ºF.
Place the eggplant, zucchini, and potatoes on separate baking sheets (I use a large one for the eggplant and two smaller ones for the zucchini and potatoes). Drizzle each with a tablespoon or two of olive oil and a few good pinches of salt, and toss to coat. Spread the vegetables in an even layer, and roast until they're all golden and tender, flipping the vegetables when their bottoms have caramelized and they will release themselves from the pan when a thin, metal spatula is involved. The zucchini will take about 20 minutes, the potatoes and eggplant 30-40 minutes.
Meanwhile, place the bell peppers over an open, medium flame and roast, turning frequently with a pair of tongs, until the skin is blackened and blistered all over. Let the peppers cool until handleable, then peel away and discard the skin, stem and seeds. Slice the peppers lengthwise into quarters, then cut the quarters crosswise into 1/4" thick slices.
Meanwhile, toast the cumin seeds in a dry skillet over a medium flame until fragrant, 30-60 seconds. Let cool, then grind in a mortar and pestle or spice grinder.
Peel the onions, halve them, then slice them thinly. Warm 3 tablespoons olive oil in a large soup pot or dutch oven over medium heat. Add the onions, ground cumin, sweet and smoked paprikas, turmeric, ginger, cinnamon stick, saffron, and 1/2 teaspoon salt. Cook, stirring frequently, until the onions are tender, 5 minutes. Add the garlic, lower the heat a bit, and continue cooking until the onions are very tender, 5-10 minutes more.
Meanwhile, slice the tomatoes in half horizontally. Squeeze the seeds out into a sieve placed over a bowl to catch the juices, and work the pulp to extract as much juice as possible. Discard the seeds. Coarsely chop the walls of the tomatoes.
When the onions are ready, drain the chickpeas, reserving the broth and discarding the bay leaf. Add the chickpeas to the pot of onions along with 4 cups of their broth (if you don't have enough, make up the difference with water). Add the roasted eggplants, zucchini, potatoes and bell peppers, and the chopped tomatoes and their juice. Bring the tagine to a bare simmer, and cook gently, stirring once or twice, until the tomatoes are beginning to break down, 10-15 minutes.