Preheat oven to 400 degrees F.
In a large cast iron skillet melt butter over medium low heat. Add onions and allow them to cook till they soften and begin to turn brown around the edges. I cooked mine for about 10 minutes. If you cook on higher heat it can be done in about 5 to 7 minutes. Remove onions from the pan and set aside.
In the same pan brown the ground sausage until it's cooked thoroughly and there's no pink left. Make sure to break it up as you cook so the pieces are quite small.
Once the ground sausage is cooked add the onions back in the pan, and add the roasted red peppers too. Cook over medium heat for about 3 to 5 minutes, just enough to combine everything and really let the flavours mingle and get to know one another.
Remove 2/3 of the mixture and transfer it to a bowl. Now it's time to layer the dip.
You have 1/3 of the sausage mixture still in the bottom of the pan. Now add a third of your Tex-Mex cheese over top, then a third of your mozzarella over that. Begin the second layer with the sausage mixture, then the Tex-Mex cheese, then the mozzarella cheese. Finish with the last layer by placing the sausage first, then the Tex-Mex cheese, then the mozzarella cheese. If there's any oils left in the bowl bowl it over top of the cheese, there's tons of flavour in that oil.
Place the pan in the oven and cook for about 8 to 10 minutes. If you like yours super crispy you can cook it for about 12 minutes. I took mine out around the 10 minute mark. While the cheese is melting cook your sausages!!