1. Center a rack in oven and heat to 350 degrees.

2. Meanwhile, bring cauliflower and broth to a simmer in a Dutch oven over medium-high. Cook, stirring occasionally, until cauliflower is tender but still holds its shape, about 10 minutes. Drain, reserving 1 cup cooking liquid, then pat cauliflower dry to remove excess moisture and set aside.

3. Add flour, butter, and garlic powder to Dutch oven and cook over medium, stirring constantly, until golden, 1 to 2 minutes. Whisk in sour cream and reserved cooking liquid and cook, whisking frequently, until smooth and creamy, about 2 more minutes; remove from heat.

4. Add cauliflower, cheese, thyme, salt, and pepper and stir until well combined. Transfer to a 9x13" baking dish and top evenly with crackers, then bake until casserole is bubbling around edges and topping is lightly browned, about 20 minutes. Let cool 10 minutes before serving.