Heat a small skillet over medium-low heat and add the walnuts, butter, 2 tablespoons of the olive oil, the 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 the remaining 3 tablespoons olive oil to the skillet to slow the cooking, and set aside.
When ready to serve, toss the greens with a little salt and pepper. Add the lemon juice and vinegar to the walnut mixture and spoon the nuts over the salad, adding as much of the dressing as needed to lightly coat the leaves. Add the blue cheese, and toss again.