Brown beef and onion in a deep 12-inch skillet over medium-high heat for 7 to 8 minutes until the meat reaches 160°F. Drain excess fat.
Add peppers and cook for 4 minutes, stirring. Add garlic and dry orzo and stir for 1 minute.
Pour in tomatoes and broth, then add salt, oregano and pepper. Bring to a gentle simmer, cover and cook over low heat for 10 to 12 minutes, stirring once halfway, until the orzo is tender.
Uncover and simmer for 2 minutes if excess liquid remains. Scatter mozzarella over the top, cover and remove from the heat for 2 minutes to melt it.
Stir gently from the bottom and serve with the pepper chunks, beef and pasta evenly distributed.