Bring a large pot of water to a boil and the cook the pasta according to the package directions. Reserve 1 cup of the pasta water before draining, then drain the pasta and set it aside.
Place the bacon in a large cast iron skillet over medium heat and cook the bacon on both sides until brown and crispy. Remove the bacon from the skillet and set it to cool on a paper towel lined plate. Once the bacon is cool, break it into bite sized pieces.
Add the thinly sliced onion into the cast iron skillet with the bacon grease and saute over medium high heat until the onions are caramelized, about 5 minutes. Add the garlic and toss for about 30 seconds until fragrant.
Add the cubed brie into the cast iron skillet along with ½ cup of the reserved pasta water and a few squeezes of lemon juice. Stir constantly until the cheese is melted and then add the cooked pasta to the mixture and toss to combine. Add the bacon and basil and season with salt and pepper as needed. Serve immediately.