1 pound russet potatoes, peeled, cut into 1-inch squares
Oil, for deep-frying,
Rendered beef fat
Sea salt flakes
2 teaspoons chopped fresh thyme
2 teaspoons chopped rosemary leaves
For the rendered beef fat:
2 pounds dry aged beef fat, coarsely chopped
2 cloves garlic
6 sprigs fresh rosemary
2 sprigs fresh sage
Cut and marinate flank steak: Cut flank steak across the grain into approximately 6-ounce portions.
In medium bowl, whisk oil, lemon zest, oregano and rosemary. Add flank steaks and turn to coat with marinade. Cover with plastic wrap and marinate at room temperature for at least 10 minutes or in refrigerator for up to 10 hours.
About 30 minutes before grilling, remove marinated flank steaks from refrigerator and let stand at room temperature.
Cook flank steak and plate: Preheat grill pan to medium-high heat.
Just before cooking, pat steak dry with paper towels and sprinkle with kosher salt and pepper. Grill steaks for 3 minutes per side, or until nicely charred and internal temperature reaches 140 degrees.
Transfer steaks to a rack set on baking sheet and let rest for 5 minutes.
Prepare fried potatoes: Make rendered beef fat. Heat fat over low heat until just beginning to melt. Add garlic, rosemary and sage. Melt until fat and solids have separated. Solids will rise to surface. Strain through chinois lined with cheesecloth. Do not force fat through cheesecloth. Discard solids. Return fat to large saucepan.
Cook the potatoes. Bring large pot of salted boiling water to a boil. Add potatoes and cook until potatoes are cooked through and tender, but not falling apart.
Drain potatoes and arrange in single layer on baking tray lined with kitchen paper. Cool completely.
Bring rendered beef fat to 275 degrees. Blanch potatoes in fat for 12 minutes. Do not overcrowd saucepan.
Using a spider, remove potatoes from fat and let cool on baking tray.
Increase rendered beef fat to 375 degrees.
Fry potatoes 3 to 4 minutes or until golden brown.
Using a spider, remove potatoes from fat and transfer to a bowl.
Toss fries with sea salt, chopped thyme and rosemary. Set aside.
Serve: Once rested, slice steaks against the grain. Divide the steak between 2 plates and sprinkle with sea salt flakes. Serve immediately with fried potatoes.