01 - Bring a large pot of salted water to a boil. Cook pasta until al dente following package instructions. Reserve 1 cup pasta water, then drain.
02 - In a large skillet over medium-low heat, warm olive oil. Add sliced shallots and cook, stirring regularly, for 15–20 minutes until deeply golden and caramelized.
03 - Add sliced garlic, red pepper flakes, salt, and black pepper to the pan. Cook for 2–3 minutes until the mixture is fragrant.
04 - Stir in tomato paste and cook for 2 minutes, allowing it to darken and coat the shallots fully.
05 - Pour in balsamic vinegar and stir thoroughly to deglaze the skillet.
06 - Add drained pasta to the skillet, tossing to coat evenly. Gradually splash in reserved pasta water as needed to create a silky sauce.
07 - Fold in Parmesan cheese, butter if using, and chopped parsley. Toss until the cheese melts and sauce embraces the pasta. Dish up immediately, garnished with extra Parmesan and parsley.