Place potatoes in a medium saucepan, add enough water to cover. Bring to boil, cook until tender, approximately 12 minutes. Drain. Cool in a bowl with cold water.
In a medium pan over low-medium heat, cook onion and garlic until tender.
Remove cooled potatoes from water. Cut each potato in half lengthwise. With a small spoon or melon baller, carefully remove small amount of center. Leaving ¼ inch rim around each potato. Place reserved potato in a small bowl and set to the side.
Add Cotija cheese, cooked onions and garlic, sour cream, and salt to the reserved potato. Mix with a fork, pressing firmly to mash.
Spoon a small amount of mixture into each potato half and place on a baking sheet.
Bake for 12 minutes or until cheese begins to brown. Top each potato with jalapeño, hard-boiled egg pieces and a sprinkle of fresh cracked pepper.