Split peppers lengthwise in half remove all seeds and discard. LIghtly coat with olive oil place in Air Fryer at 390 degrees for 15 minutes.
Meanwhile, melt 1 2 tbsp over medium-high heat. Add couscous and a pinch of salt and pepper. Toast couscous, 2-3 minutes. Add stock bring to a boil, then cover and reduce to a low simmer. Cook 6-8 minutes until couscous is tender. Turn off heat and keep covered.
While couscous cooks, dice onion and tomato. Heat olive oil in cast iron pan. Add onion and cook until softened, 4-5 minutes. Add ground beef, Italian Seasoning, salt and pepper. Cook, 4-5 minutes, then drain excess grease from your pan, by carefully pouring it out.
Stir tomato paste and diced tomato with beef mixture until thoroughly combined. Add stock and simmer until slightly thickened, 1-2 minutes. Stir in couscous until thoroughly combined and turn off heat.
Once bell peppers are done, remove from Air Fryer with tongs and stuff each half with as much filling as will fit. Place in cast iron skillet with remaining filling, nestling each stuffed pepper half into the mixture. Sprinkle evenly with mozzarella.
Bake stuffed peppers on middle rack on broil, until cheese has melted, 3-4 minutes. Serve hot from the cast iron skillet, oven to table.