However you want. As if the Mc heard about it after the fact like in Shawshank is fine, or switching to third person is fine. Or switching narrators might be an option.
Honestly, i think it’s structurally fine to just have the MC narrate the scene without breaking stride. If the structure is sound, the worst that can happen is the audience will ask how the MC has that information, but I think most will understand that if the MC is narrating, then the MC came across the info at some point between the time the story took place and the time the character began narrating that info to the audience.
How you narrate the story, or maybe I should say which character you use to narrate it, seems like a storytelling choice with infinite possibilities. Structurally, I have to think that the Storymimd itself is the narrator no matter which character it’s narrating through. Structurally, your Storymind is relating it’s already complete process of exploring the problem to the audience and thus already has all the info necessary to tell the story, no explanation needed.