Make potatoes: Preheat oven to 400 degrees F and arrange rack in center. Cut 3 squares of foil, each large enough to wrap around 1 potato. Prick each potato in 3 places with a fork and place on foil. Drizzle each with 1 teaspoon oil and wrap tightly. Place in a baking pan and roast, turning once, until very tender, 1 to 1-1/4 hours. Transfer potatoes in pan to rack and let cool slightly. Carefully unwrap potatoes, halve, peel and mash in a bowl. Add remaining ingredients and mash together until smooth; salt and pepper to taste. Keep warm, covered.

Make sauce: Meanwhile, whisk together all ingredients in a bowl; salt and pepper to taste.

While potatoes are roasting, rinse steaks and pat dry; let stand at room temperature 30 minutes. Season with salt and pepper.

Heat oil in a large skillet over moderately high heat until hot but not smoking. Add steaks and cook, turning once, about 8 minutes for medium-rare (reducing heat to moderate if smoking). Transfer to a cutting board and let stand 5 minutes before cutting into 1/4-inch-thick slices.

Arrange steak alongside potatoes on serving plates. Spoon some sauce over steak and serve remaining on the side.