Lemons, salt. So far that's it. I'll let the salt suck as much juice as it can out of the lemons overnight, and then I'll top it off with water. Then the recipe says 6 weeks in the fridge. I doubt there will be much bacterial action in this case, especially if I chill it the whole time. If it does ferment, it shouldn't produce much gas, since it's already so acidic and the brine will be nearly saturated.
I'm thinking about putting it in the basement instead (~60 F instead of <40) but I've deviated from the recipe enough already; it called for more lemons to be juiced in and WAY more salt. It also recommended to keep the lemons just barely whole, cut into wedges but not quite all the way through. I was never going to get 5 lemons in a quart jar like that. Maybe if I was making 2 dozen in a bucket for market in Casablanca.
This is not going to be a snack, I think it's meant to be more of a flavoring for a baked meat dish. But in that context it's going to be amazing. Also the brine is going to be a trip. Super-salty 6-month-old lemon juice? Yes, please.