1. Preheat broiler to high. Heat oil in a large ovenproof skillet over medium-high. Add onion; sauté 2 minutes. Add beef and oregano; cook 3 minutes, stirring to crumble.
2. Add stock, salt, pepper, tomatoes, and pasta; bring to a boil. Cover, reduce heat, and simmer 12 to 13 minutes or until pasta is done. Stir in spinach.
3. Sprinkle cheese over pasta mixture. Place pan in oven; broil 2 minutes or until cheese melts and begins to brown.