Heat olive oil in a large pot over medium-high heat
Add Italian sausage and cook until browned, breaking it apart as it cooks
Add butter and sliced leek to the pot, season with salt and pepper, and cook until leek is softened
Stir in minced garlic and red pepper flakes, cooking until fragrant
Deglaze the pot with dry white wine if using
Add chicken broth and pasta to the pot
Simmer until pasta is cooked and sauce begins to build
Stir in heavy cream and frozen peas
Add grated Parmesan, lemon juice, and Dijon mustard if using
Taste and adjust seasoning with salt and pepper
Garnish with fresh chives or parsley and additional Parmesan before serving