Preheat oven to 375°F and grease a 9x13 pan.
Heat oil in a skillet over medium. Sauté onion until soft, 4–5 minutes. Add garlic and cook 30 seconds.
Stir in thawed hashbrowns and riced cauliflower. Cook 3–4 minutes to remove excess water.
Mix Greek yogurt, chicken broth, cheddar, Parmesan, salt, pepper, and thyme in a bowl.
Combine hot veg with the cheese sauce and stir well. Transfer to pan and smooth top.
Sprinkle panko if using. Bake 25–30 minutes until bubbly and golden on top. Let rest 5 minutes before serving.