Bring a large pot of salted water to a boil. Add the pasta and prepare according to directions. Drain and set aside.
Heat a large skillet with high sides over medium-high heat and cook bacon until crispy. Remove to a paper towel-lined plate. Remove all but one tablespoon of bacon fat from the pan and then add 1 tablespoon olive oil over high heat. Add garlic and saute for 30 seconds. Add the brussels sprouts in a single layer and sprinkle with brown sugar and salt. Let sit 1 minute and stir. Continue to saute for another 4-5 minutes, until bright green and tender. Now add the pecans and cook another 3-5 minutes. Remove everything from the pan and carefully wipe the pan clean.
To the same pan add the butter, milk and mascarpone cheese. Bring to a boil and simmer until smooth and creamy, about 5 minutes. Add the parmesan, parsley, crushed red pepper, pepper, salt and nutmeg, whisking until the cheese has melted. Simmer the sauce 3-5 minutes until the sauce is smooth and creamy. Add the pasta and brussels sprouts to the sauce and toss well, cook another 1-2 minutes to warm through. Serve immediately with fresh parmesan.
If your skillet is not big enough to toss the pasta in transfer everything to a large pot or serving bowl and toss well.
*This can easily be made vegetarian by leaving out the bacon. Just add a tablespoon more of olive oil when caramelizing the brussels sprouts.