In a medium saucepan, combine the wild rice mix and 3 cups of the chicken broth. Bring to a boil over medium heat, cover, reduce the heat to medium-low, and cook until the rice is tender and the broth is absorbed, about 30 minutes.
Meanwhile, in a large saucepan, melt the butter over medium heat. Add the onion and garlic and sauté until fragrant and slightly browned, about 3 minutes. Sprinkle in the flour and cook for 2 minutes to make a roux. Slowly stir in the milk and remaining 1 cup broth and stir until smooth and starting to thicken.
Preheat the oven to 350 degrees. Spray a 9×3 inch baking dish with cooking spray.
Add the cream cheese and 1 cup of the cheddar to the roux and stir until the cheese is melted. Add the garlic salt, kosher salt, pepper, broccoli, and cooked wild rice and stir until well combined.
Pour the broccoli mixture into the prepared baking dish and top with the remaining 1 cup cheddar. Bake until the cheese is melted and bubbling, 15 to 20 minutes. If you want a more browned top, broil the casserole for up to 5 minutes, watching it carefully.
Store in an airtight container in the refrigerator for up to 5 days.