create the structure of the dish and absorb flavor as they bake. Onion softens between the layers and adds moisture and sweetness. Canned corned beef provides rich, salty, savory flavor without requiring any additional cooking before assembly.
Covering the casserole for the first part of the baking time traps steam, which helps the potatoes soften. Removing the foil near the end allows excess moisture to escape and gives the top layer time to brown.
That combination of steaming first and browning second is what gives the finished dish its fork-tender
