Preheat your air fryer to 350°F (175°C).
In a pan, heat olive oil and sauté the diced onion and bell pepper until softened (about 5 minutes). Set aside to cool.
In a large bowl, combine ground beef, crushed Ritz crackers, eggs, shredded cheddar cheese, whole milk, salt, pepper, and sautéed onions and bell peppers. Mix until well combined.
Shape the meat mixture into a loaf, about 9 inches long and 5 inches wide. Ensure it's compact and even for optimal cooking.
Place the meatloaf into the air fryer basket. Cook at 350°F for 20-25 minutes, checking for an internal temperature of 160°F (71°C) and brushing with more topping halfway through if desired.
While the meatloaf is cooking, combine ketchup, brown sugar, and yellow mustard in a bowl to create the topping.
Once the meatloaf reaches the desired internal temperature, remove from the air fryer and let it rest for 5 minutes before slicing. Serve warm with your favorite sides.