To make the yellow cheese sauce, heat the vegetable oil in a medium-sized frying pan over low–medium heat. Add the onion and garlic and cook until softened. Add the aji amarillo paste and cook until fragrant. Remove from the heat, allow to cool slightly, then transfer to a food processor and process until smooth. Add the queso fresco and continue to process to make a coarse paste. Gradually pour in the evaporated milk and ¼ cup water and process to make a smooth thick sauce. Transfer to a small bowl and set aside. Preheat a gas or charcoal barbecue to medium. Cut two pieces of foil, large enough to wrap the potato in two batches. Divide the potato among the foil pieces and drizzle with olive oil. Wrap to seal and enclose the potato. Place the parcels on the grill plate or bury in hot coals and cook for 30–45 minutes, turning occasionally, until the potatoes are tender. Insert a skewer through the foil and into the potatoes to test if they are done. To serve, unwrap the potato parcels and transfer to a large serving bowl. Drizzle with the prepared sauce and scatter with the boiled egg and olives. Serve hot with grilled meat, chicken or fish.