Star Trek: 10 Things You Didn’t Know About Paul Stamets
2. Mushrooms First, Coffee Later
It was during the Muddest of time loops in Magic to Make the Sanest Man Go Mad that we learnt about Stamets' first encounter with his future husband at a café on Alpha Centauri. Burnham hit the Kasseelian high note on the head when she replied, "But after such a rude exchange, why would he do that?" Good point! Telling a stranger to can it with the humming is not typically a prelude to romance.
The Star Trek: Discovery Annual 2018 also affords us a lot more detail both on the history of Paul and Hugh's relationship, and on the beginnings of the spore drive. In it, we meet pre-Starfleet-recruited Doctors Stamets and Straal as they uncover some pretty stellar properties of Prototaxites stellaviatori during their terraforming work on the planet Deneva (think Spock attack from the back, but don't tell Sam Kirk).
Two years later, in the comic, the duo present their research on instant matter transfer across unlimited cosmic distances via the mycelial network in front of their decidedly (and understandably) incredulous peers at the Federation Research Council on Alpha Centauri. Starfleet is more convinced, however, and after several more months of research (with human test subjects including Stamets himself), Stamets and Straal are given two experimental ships, one of which we spent five seasons with, of course.
As for the coffee 'first date,' in the Annual, Stamets is seemingly on the much harder stuff after that disappointing research presentation. He does tell Culber that he "hates" the humming, but the latter calls him "rude" and walks away. The pair bump into each other, however, before leaving Alpha Centauri, and the rest is almost a thousand years of history.