In a large skillet over medium-high heat, melt butter then whisk in flour until smooth. Slowly add milk, whisking constantly until smooth. Whisk in half-and-half, pureed roasted red peppers, and mustard. Bring sauce to a simmer over medium-high heat until bubbly and slightly thickened.
Remove sauce from heat and whisk in cheese and salt and pepper to taste. Stir in cooked pasta and chicken sausage slices. Return mac 'n' cheese to medium heat and stir occasionally until heated through. Garnish mac 'n' cheese with fresh basil and serve immediately. Enjoy!