Cook the linguine in salted boiling water until al dente. Reserve ¼ cup of the pasta cooking water, then drain the pasta.
Season the cubed chicken breasts with garlic powder, onion powder, smoked paprika, cayenne pepper (if using), salt, and pepper. Heat olive oil in a skillet and cook the chicken until golden brown and fully cooked through. Remove from the pan and set aside.
In the same skillet, melt the butter. Add chicken broth, heavy cream, Dijon mustard, and crushed red pepper flakes (if using). Stir and let the sauce simmer until it thickens slightly.
Return the cooked chicken and the drained linguine to the skillet, tossing everything together to fully coat with the sauce. Stir in the Parmesan cheese, chopped parsley, and lemon juice. If needed, adjust the sauce consistency by adding some of the reserved pasta water.
Garnish with additional parsley and Parmesan cheese before serving. Serve warm.