Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish, or prepare a large oven-safe skillet.
Boil the fettuccine pasta in salted water for 2 minutes less than the package instructions for al dente. Drain well and set aside.
Heat olive oil in a large oven-safe skillet over medium-high heat. Season the chicken pieces with salt, black pepper, and half of the smoked paprika. Sear until golden and cooked through, about 6 to 8 minutes. Remove the chicken from the skillet.
Reduce the heat to medium and melt the butter in the same skillet. Add the diced onion and minced garlic, sautéing until softened and fragrant, about 3 to 4 minutes.
Whisk in the Worcestershire sauce, Dijon mustard, lemon juice, remaining smoked paprika, cayenne pepper, and red pepper flakes. Cook for 1 minute.
Pour in the heavy cream and bring to a gentle simmer. Remove from heat and stir in half of the grated Parmesan cheese until the sauce is smooth.
Return the cooked chicken and drained pasta to the skillet, tossing until evenly coated. If not using an oven-safe skillet, transfer the mixture to the prepared baking dish.
Top evenly with the remaining Parmesan cheese.
Bake for 15 to 20 minutes until the cheese is melted and bubbly. Garnish with chopped fresh parsley before serving.