Preheat oven to 400°F (200°C).
Rinse and drain one can of chickpeas (15 ounces). Pat the chickpeas very dry on paper towels, remove any loose skins, and place in a large bowl.
Toss the dried chickpeas with 1 tablespoon of the 2 1/2 tablespoons olive oil and a pinch of black pepper. Lightly season with part of the 1/2 teaspoon salt from the ingredient list, reserving the remainder of that salt to add later when blending.
Spread the oiled chickpeas in a single layer on a large baking sheet (don’t overcrowd). Bake 20 minutes, stir, then bake an additional 5–10 minutes until the chickpeas are very crispy. Let cool slightly.
Transfer the roasted chickpeas to a food processor and pulse until broken down and crumbly but not powdered. You should retain some texture. Scoop out 1/2 cup of the roasted chickpea crumbs for the fritter mixture and set the rest aside for another use.
Heat the remaining 1 1/2 tablespoons olive oil in a large skillet over medium-high heat. Add the chopped onion (1 cup, about 1/2 a small onion) and the minced garlic (2 tablespoons). Cook, stirring, about 2 minutes until the onion and garlic are lightly golden.
Add the chopped cauliflower (2 cups, about 1/2 a large head) to the skillet and cook 2 more minutes until the cauliflower begins to brown. Reduce heat to low, cover the skillet, and cook 3–5 minutes more, stirring occasionally, until the cauliflower is fork-tender and the onions are caramelized.
Transfer the cooked cauliflower and onion mixture to the food processor. Drain and rinse the second can of chickpeas (15 ounces) and add them to the processor along with the remainder of the 1/2 teaspoon salt and a pinch of black pepper. Process until the mixture is smooth and starts to come together into a ball, stopping to scrape down the sides as needed.
Transfer the blended cauliflower–chickpea mixture to a large bowl and stir in the reserved 1/2 cup roasted chickpea crumbs until well combined. Use your hands to form the mixture into patties (work in a size that fits your skillet and preference).
Pour just enough oil into a large skillet to lightly cover the bottom and heat over medium. Fry the patties in batches, 2–3 minutes per side, until golden brown and heated through. Add more oil between batches as needed and avoid crowding the pan.
Transfer cooked fritters to a plate lined with folded paper towels to drain briefly. Top each fritter with hummus of choice and garnish with diced green onion. Serve immediately.