Preheat oven to 425°.
On a rimmed baking sheet, toss sweet potato pieces in olive oil. Season with salt and pepper, and roast, turning over halfway through roasting, until they are tender and browned, about 40 minutes.
While sweet potatoes are roasting, cook chickpeas. Heat olive oil in a medium skillet over medium-high heat. Once it shimmers, add chickpeas and cook 2 to 3 minutes so that oil coats chickpeas nicely. Then add cumin and salt and pepper to taste. Toss to coat the chickpeas. Then cook, tossing chickpeas frequently, about 15 minutes, or until they start to brown and crisp on the outside but are still tender on the inside.
While chickpeas and sweet potatoes are cooking, you can make the dressing! In a high-speed blender or food processor, combine avocado flesh with olive oil, garlic, cilantro and lime juice, and blend well. Add 3 to 4 tablespoons of water to the dressing to thin it out slightly. Feel free to add a couple of extra tablespoons of water to thin to your desired consistency. Season dressing with salt and pepper to taste.
To make salad, toss cabbage, roasted sweet potatoes and grape tomatoes in a large serving bowl. Add dressing and toss to coat. Sprinkle some chickpeas on top, and serve the rest on the side for easy access.