Preheat oven to 425 degrees.
Chop leftover meat and vegetables of choice in to small, bite sized pieces. Dice onion. Set to the side.
In a cast iron skillet over medium heat, melt 1/3 cup of butter. Add in diced onion and cook 5-7 minutes, or until onion begins turning translucent.
Add in 1/3 cup of flour. Stir continuously to combine thoroughly with melted butter and onion.
Slowly pour in milk and broth, whisking as you pour. Once combined, reduce heat to medium-low heat and allow to simmer for 10 minutes. The filling should thicken as it simmers.
Turn off heat. Dump leftover meat and vegetables into the filling mixture and stir to combine. Add salt and pepper to taste at this point.
If cooking in a cast iron skillet, move on to biscuit topping instructions. If cooking in non-oven safe skillet, transfer filling to a glass baking dish and then move on to biscuit topping instructions.