Pat-dry the steak, season with salt and pepper, and place in a shallow glass pan. Pour 2 cups rosemary oil over steak, cover, and refrigerate for several hours. Turn the steak occasionally to coat both sides. Allow steak to come back to room temperature before grilling. Reserve 1/4 cup oil. Place a ridged grill pan over moderately high heat until very hot. Season the steak again and grill steak about 5 minutes per side for medium rare. Remove steak to a cutting board and let stand 10 minutes before cutting in slices.
Brush the flesh of the peaches with remaining rosemary oil. Place cut side face down on the hot grill pan for 3 minutes. Turn over and grill the other side just for 1 minute to soften. Remove and slice peaches into wedges. Toss the greens and herbs together and divide among plates. Fan the steak slices and peach wedges on top. Drizzle the dressing over steak and peach salad, garnish with remaining 1/4 pound crumbled blue cheese. Season with freshly cracked black pepper and serve.
Rosemary Infused Olive Oil:
To prepare the rosemary infused olive oil: Warm oil and rosemary over low heat in a small saucepan. Allow to steep for 8 minutes. Remove from heat and cool to room temperature.
Creamy Blue Cheese Dressing:
To prepare the creamy blue cheese dressing: Whisk together 1/2 pound blue cheese with sour cream, lemon, vinegar and cayenne until smooth. Fold in chives, cover, and chill until ready to serve.
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