Bring a large pot of lightly salted water to a full boil. Add the egg noodles and cook for approximately 7–9 minutes, or until tender while still slightly firm when bitten. Drain thoroughly and set aside.
While the noodles are cooking, heat the oven to 350°F (175°C).
Place a medium skillet over medium-high heat. Add the lean ground beef, chopped onion, green bell pepper, and minced garlic. Cook while stirring and breaking apart the meat for 5–7 minutes, until the beef is browned and crumbly.
Drain away any excess fat from the skillet.
Add the cooked egg noodles, pizza sauce, sliced pepperoni, and milk to the skillet containing the beef mixture. Stir thoroughly until all ingredients are evenly incorporated.
Transfer the mixture to a 2-quart casserole dish and distribute it evenly.
Place the casserole in the preheated oven and bake for 20 minutes.
Remove or pull out the dish sufficiently to scatter the grated mozzarella evenly over the surface. Return the casserole to the oven and bake for another 5–10 minutes, until the mozzarella is fully melted and bubbling.
Remove the finished casserole from the oven and allow it to cool slightly before serving.