Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower florets, cover, and steam until tender, about 15 minutes. Drain; transfer cauliflower to a 9x13-inch baking dish.
Preheat oven to 350 degrees F (175 degrees C).
Melt butter in a large skillet over medium heat. Add onion and garlic; cook and stir until softened, 5 to 7 minutes.
Combine sour cream, Cheddar cheese, cream of chicken soup, salt, and black pepper in a large bowl; fold in onion mixture. Pour sour cream mixture over cauliflower in baking dish. Scatter cracker crumbs on top.
Bake in the preheated oven until bubbly and cheese is melted, about 20 minutes.
2 heads cauliflower, broken into florets
1 tablespoon butter
1 onion, chopped
2 garlic clove (blank)s garlic cloves, crushed
1 cup sour cream
1 cup shredded Cheddar cheese
½ (10.5 ounce) can cream of chicken soup, or more to taste
salt and ground black pepper to taste
1 cup crushed buttery round crackers (such as Ritz®)