Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and set aside to cool slightly.

Meanwhile, heat olive oil in a skillet over medium heat. Cook and stir onion in hot oil until tender, about 10 minutes. Remove from heat.

Bring a small saucepan of water to a boil. Cook chile peppers in boiling water until soft, about 5 minutes. Run chiles under cold water until cool enough to handle; remove and discard skins.

Add onion, chile peppers, cream cheese, queso fresco, vegetable oil, salt, garlic, and black pepper to a blender; blend until smooth. Stream evaporated milk into blended mixture with the blender on until sauce is creamy.

Arrange 1 lettuce leaf onto each of 8 plates. Halve potatoes and arrange 3 halves onto each lettuce leaf. Pour sauce over potatoes. Top each serving with an egg half and 2 olive halves.