broadly Wellington subscribed to eighteenth century notions that the best professional education for an officer was on campaign, attached to the staff of some great captain. Of course, he would have argued that regimental service and the study of certain written works was also necessary, but he was dubious about organisations such as the Royal Military College.
You use the word 'mentoring' and it is a good one. Those who exemplified this method were FitzRoy Somerset, Lord March or Cocks. He liked them to be from aristocratic families, but obviously also to have some ability and commitment. All three of those officers are notable for taking little or no leave during the Penninsular campaign.
Officers like George Scovell fell into a different category - they were among a small number essential to the smooth running of headquarters who had to be patronised and promoted for reasons of expediency. He did not however consider someone of Scovell's origins to be suitable for grooming as a future general.
Very occasionally someone crossed from the latter category into the former - Henry Hardinge being a good example. He was a Shropshire vicar's son and RMC graduate who Wellington eventually to the highest offices of state.