Filled with fresh zucchini and squash, this casserole doesn't require the oven.
Process tomatillos, green chiles, cilantro stems, garlic, vinegar, salt, and pepper in a blender until mostly smooth, about 15 seconds.
Place rice on bottom of a 5- to 6-quart slow cooker. Top with squash, zucchini, and tomatillo sauce. Cover and cook on LOW until squash is tender and rice is cooked, about 5 hours.
Reduce slow-cooker heat to WARM; add sour cream and 3 cups of the cheese, and stir until cheese is melted. Sprinkle with cilantro leaves and remaining 1 cup cheese just before serving.