Preheat oven to 425 degrees F.
Prepare all vegetables and place in a large bowl. Cover generously with olive oil and Fennel Spice Rub. Cover and toss well to combine.
Heat a very large ovenproof skillet over high heat. Add 1/4 cup of the olive oil. When the oil is hot, add the carrots and cook for about 1 minute, then add the onions and cook, turning occasionally with tongs, until the vegetables are nicely browned, about 10 minutes. Reduce the heat if needed to keep them from burning.
Add the fennel bulb, zucchini, additional Fennel Spice Rub or crushed fennel seed, and salt, to taste. Toss well to distribute the seasonings. Drizzle with the remaining 1 tablespoon oil and toss again. Transfer the skillet to the oven and roast until the vegetables are deeply caramelized, 20 to 25 minutes, stirring them occasionally so they cook evenly. Serve immediately.
Put the fennel seeds, coriander seeds, and peppercorns in a heavy pan over medium heat. Watch carefully, tossing frequently so the seeds toast evenly. When light brown and fragrant, pour the seeds onto a plate to cool. They must be cool before grinding, or they will gum up the blades.
Pour the seeds into a blender and add the salt. Blend to a fine powder, shaking the blender occasionally to redistribute the seeds. Store in a tightly sealed glass jar in a cool, dry place, or freeze.