Preheat oven to 375℉.

Add butter to a small skillet and heat over medium high heat. Add diced bell pepper and onions. Sauté until peppers are softened and onions become translucent. This will take about 10 minutes.

While the onions and peppers are doing their thing, cut the stick of pepperoni into chunks and add to a food processor with the blade attachment. Pulse until ground consistency. 

In a mixing bowl add the ground pepperoni, ground beef, and ground sausage. Use a spatula to break the meat up and give it a stir to mix it up.

Add the peppers, onions and that melted butter it cooked in over the meat mixture. Give a quick mix with the spatula to combine.

Add cheese, tomato sauce, seasonings, and beaten eggs. Give a stir to combine. 

Use hands to mix everything until well combined.

Add the meatloaf mixture to a large casserole dish and form into a loaf.

Place the meatloaf into your preheated oven and bake for 1 hour and 15 minutes. The last 15 minutes of baking add tomato paste topping if desired.