Brown the beef with the onions and salt and pepper to taste in a 10- or 12-inch cast iron skillet (or other oven safe skillet) over medium-high heat.
Turn off heat. Add frozen vegetables on top (no need to defrost).
Use an offset spatula to spread the mashed cauliflower on top of the vegetables. Drizzle melted butter on top.
Bake at 400 degrees F for about 20 minutes, or until heated through. Broil for another 5-10 if not browned sufficiently.
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