Place bacon in a large skillet and cook over medium high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels.
Whisk mayonnaise, Parmesan cheese, sugar, and basil together in a large bowl until smooth. Add broccoli, cauliflower, and red onion to mayonnaise mixture; toss to coat. Refrigerate salad until chilled and flavors have blended, 3 hours to overnight.
Crumble bacon. Mix bacon, lettuce, croutons, and water chestnuts into salad.
Broccoli and Cauliflower Salad · Dinner Match Lab | Dinner Match Lab