Cook pasta according to package directions, omitting salt and fat. Drain well; set aside.
Heat oil in a large nonstick skillet over medium heat until hot. Add pancetta and sauté for 3 minutes. Stir in onion, ¼ teaspoon salt, and ⅛ teaspoon pepper; cover and cook 15 minutes, stirring frequently. Uncover and cook 10 additional minutes, stirring frequently. Remove onion mixture from skillet; set aside.
Add mushrooms, ¼ teaspoon salt, and ⅛ teaspoon pepper to skillet; cook over medium high heat 4 minutes. Add minced oregano and 2 tablespoons parsley; cook 1 minute. Add garlic; sauté 1 minute. Add onion mixture and broth; cook until thoroughly heated. Add mushroom mixture to pasta; toss to coat.
Divide among 4 shallow bowls; sprinkle evenly with 2 tablespoons parsley. Garnish with fresh oregano sprigs, if desired.
Notes
Pancetta is Italian bacon that is cured with salt and spices but not smoked. Regular bacon or ham can be substituted for it.