Preparation

• Position a rack about 8 inches from the broiler, and heat the broiler on high. In a 12-inch skillet, heat the oil over medium-high heat. Add the sausage and cook, stirring occasionally and breaking it apart, until golden brown in places, about 3 minutes. With a slotted spoon, transfer the sausage to a medium bowl. Add the mushrooms, onion, rosemary, and 1/2 tsp. salt to the skillet. Cover and cook, stirring once, until the mushrooms are golden, about 5 minutes. Add the beer, scrape up the browned bits on the bottom of the pan, and simmer until the beer evaporates, about 1 minute. Toss with the sausage.
• Meanwhile, toast the baguette on a sheet pan, cut side up, under the broiler until lightly golden and crisp, 1 to 2 minutes.
• Spread the bread with the mascarpone, and lightly season with salt and pepper. Top with the sausage mixture and then the mozzarella. Broil until the cheese melts, about 3 minutes. Sprinkle with the parsley, cool briefly, and serve.