1. Place porcini and broth in small microwave-safe bowl; cover with plastic wrap and cut several steam vents in plastic with paring knife. Microwave on high power 1 minute, until broth is steaming. Let stand until mushrooms soften, about 10 minutes. Lift mushrooms from broth with fork and finely chop. Strain broth through fine-mesh strainer lined with large coffee filter into medium bowl. Set aside mushrooms and broth.

2. Heat pancetta in 12-inch skillet over medium heat; cook, stirring occasionally, until rendered and crisp, 7 to 10 minutes. Add portobellos, chopped porcini, olive oil, garlic, tomato paste, and rosemary; cook, stirring occasionally, until all liquid has evaporated and tomato paste starts to brown, 5 to 7 minutes. Add reserved chicken broth, crushed tomatoes, and their juices; increase heat to high and bring to simmer. Reduce heat to medium-low and simmer until thickened, 15 to 20 minutes. Season with salt and pepper to taste.

3. While sauce simmers, bring 4 quarts water to boil in large Dutch oven. Add 1 tablespoon salt and pasta; cook until al dente. Drain pasta, reserving 1/2 cup cooking water, and return to pot. Add sauce to pasta and toss to combine. Adjust consistency with reserved pasta water and season with salt and pepper to taste. Serve, passing Pecorino separately.