Has the steel been designed to allow for the door to be hung off it, how much will it deflect once the door is hanging off it?
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Sorry I didn't realise you had 30cm to build up You will still need something like 6x2 under your lintel to catch the insulation and plasterboard so I would box it out in that timber and either osb or ply on the inside and as cdbe says pvc or cement board on the outside.

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