While split brain research has clearly shown that the brain has the capability to construct such narratives even when the real source of the decision is entirely different, I don't agree that it's well-established that under normal conditions such narratives are only constructed after the decision is made. I think it's more likely that the process of creating the narrative is more integrated with the process of making the decision, even if the precise sequence is different than what we experience consciously (that is, what the constructed narrative says it was).
Of course, "making a decision" is something that only happens on that abstract narrative level. Nothing in nature makes a decision; neither rocks, hurricanes, digital circuitry, nor neurons act on anything besides cause and effect and quantum randomness. So how can "your mind" make a decision? Because your mind happens on that abstract narrative level too, as a character in the narrative your brain constructs. Superman can fly; your mind can make decisions.