Preheat oven to 400 degrees. Preheat the cast iron skillet on the stove top.
Cook the meat in the skillet till completyly cooked. (If using leftovers, skip this step.)
Remove the cooked meat from the skillet and set aside. (Also remove any excess fat that may be left in the skillet
Add 2 tbsp oil to the skillet
Saute the vegetables in the skillet. (If using onions, cook those first till soft. Then add the rest of your vegetables to finish cooking.) This should take about 6 minutes on medium heat.
While the vegetables cook, mix together the gluten-free sourdough bread topping. (But don't forget to continue to stir vegetables in the skillet occasionally too).
Add all topping ingredients, except for the cheese, to a large mixing bowl.
Whisk until well combined. Set aside.
Add the cooked meat to the skillet filled with the sauteed vegetables.
Add 3 cloves of minced garlic, stir to combine.
Cook for 30 seconds.
Make sure the filling mixture is evenly distributed in the skillet.
Remove the cast iron skillet from the heat.
Pour the gluten-free sourdough bread topping over the filling mixture.
Use a spatula to make sure it is also evenly spread over everything.
Transfer the skillet into the preheated oven.
Bake for 20 minutes.
Remove the skillet.
Top with the 1 1/2 cups shredded cheese.
Return to the oven for 5 more minutes, until the cheese is melted.