All in one method for me....
Peel a few potatoes (as many as you want) largish chunks and place in baking tray with chunks of onion, carrott and any other veg you want. Pour a little oil over, add about an inch of water and crumble an oxo cube over, cover with a sheet of foil and put in oven about 180 for an hour giving it the odd shake. (This bit is like par boiling, when the water has evaporated its starts to roast in the oil) Remove foil, give it a good toss round, place cubed corned beef on top and cook some more. I like my CB to go a bit toasty on the edges, I also just do mine with just the tats and onion no over veg and cover the CB in grated cheese stick back in oven till that has melted and you have a wonderful gooey extremely tasty mess. My kids love this 13 and 5 made with baked beans, before you add CB add beans to tin on top of tats and onion, CB then cheese. All in one roasting dish!! works every time