Creamy Cajun Shrimp and Sausage Pasta

Name: Creamy Cajun Shrimp and Sausage Pasta
Ingredients: 1/4 cup unsalted butter; 1/4 cup all-purpose flour; 1 medium onion, chopped; 1 cup chopped celery; 2/3 cup chopped red bell pepper; 2/3 cup chopped green bell pepper; 2 cloves garlic, minced; 2 teaspoons Cajun seasoning; 1/2 cup chicken broth; 1/2 pound thinly-sliced, fully-cooked smoked sausage; 1/4 teaspoon crushed red pepper flakes; 1/2 teaspoon ground black pepper; Salt, to taste, see Note; 1/2 pound raw shrimp, peeled, deveined, see Note; 1/2 cup cream; 12 ounces spaghetti, cooked; Fresh parsley for optional garnish; Lemon slices for optional garnish