In a covered medium saucepan cook potatoes in a small amount of boiling salted water about 10 minutes or until nearly tender. Drain well.

Meanwhile, in a small bowl combine chili powder, cumin, garlic powder, salt, oregano, and cayenne pepper. Gently toss potatoes with 2 tablespoons of the lemon juice and the oil. Sprinkle with 1 1/2 teaspoons of the chili powder mixture. Toss again to coat potatoes evenly. Arrange potatoes in a single layer in a lightly greased 15x10x1-inch baking pan; set aside.

Brush both sides of roast with remaining 1 tablespoon lemon juice. Sprinkle with remaining chili powder mixture. Place roast on a rack set in a shallow roasting pan. Insert a meat thermometer into center of roast. Roast, uncovered, in a 425 degree oven until desired doneness. Allow 30 to 35 minutes for medium rare (140 degrees F) or 40 to 45 minutes for medium (155 degrees F). Remove roast from oven. Cover with foil and let stand for 15 minutes. (The meat's temperature will rise 5 derees F while it stands.)

Meanwhile, while roast is standing, place the potatoes in the 425 degree F oven. Bake, uncovered, about 15 minutes or until potatoes are tender and brown, carefully turning potatoes once.

To serve, thinly slice roast across the grain. Serve potatoes with roast. 6 to 8 servings.