Prepare the steak: Heat the olive oil and rosemary in a small saucepan over low heat, 8 minutes. Remove from the heat and let cool to room temperature. Pat the steak dry, season with salt and black pepper and place in a shallow glass pan. Pour 1 cup of the infused oil over the steak; cover and refrigerate 3 hours, turning the steak occasionally. Remove from the refrigerator about 45 minutes before cooking.
Heat a grill pan over medium-high heat until very hot. Season the steak with salt and black pepper and grill 5 minutes per side for medium rare. Remove to a cutting board and let rest 10 minutes before slicing.
Make the dressing: Whisk half of the blue cheese with the sour cream, lemon juice, vinegar and cayenne in a bowl until smooth. Fold in the chives; cover and chill until ready to serve.
Make the salad: Brush the cut side of the peaches with the remaining 1/4 cup infused oil. Heat the grill pan over medium-high heat. Grill the peaches, cut-side down, 3 minutes. Flip and grill the other side 1 minute to soften. Remove and slice into wedges. Toss the greens and herbs in a large bowl and divide among plates. Top with the steak and peaches; drizzle with the dressing and top with the remaining blue cheese. Season with black pepper.
Photograph by David Malosh
Recipe courtesy Tyler Florence for Food Network Magazine