In a non-stick skillet over medium heat, melt the butter. Add the brown sugar and walnuts. Stir occasionally until sugar has melted and nuts have started to lightly toast, about 5 minutes. Transfer nuts to parchment paper and allow to cool completely.
In a jar, combine the vinegar, honey, citrus, garlic, salt, pepper and oil. Shake to combine.
Divide the lettuce among four shallow bowls (2 cups each). Top the lettuce evenly with the grapes, sugared walnuts and blue cheese. Dress with the citrus vinaigrette and serve.