Preheat oven to 375 degrees F (190 degrees C). Line a baking sheet with aluminum foil.
Whisk brown sugar and water together in a small saucepan; bring to a boil and cook until reduced to a caramel syrup, 5 to 10 minutes.
Arrange walnuts and pecans on the prepared baking sheet.
Toast nuts in the preheated oven until fragrant, about 5 minutes. Pour caramel syrup over nuts; toss to coat. Bake until nuts are lightly browned and syrup has thickened, 8 to 10 more minutes. Let cool on baking sheet.
Mix salad greens, strawberries, blackberries, blue cheese, cranberries, dates, and vinaigrette dressing; toss to coat. Sprinkle candied nuts over salad.