Heat a medium skillet over medium-low heat and add the walnuts, butter, olive oil, garlic, rosemary, salt and pepper. Cook very gently, stirring often, until the walnuts turn golden brown and the garlic has cooked, about 10 minutes.
Add maple syrup, lemon juice, and balsamic vinegar.
When ready to serve, spoon the nuts over the salad, adding as much of the dressing as needed to lightly coat the leaves. If desired, sprinkle blue cheese over the top of the salad.