I’ve been making this recipe from Alton Brown for pickled mushrooms:
Last time I made this, I used baby bellas from Costco, and made two batches. I used the 3/4 teaspoon sugar called for in the recipe for one batch and 1/4 teaspoon of sugar for the second batch. I liked the second batch (1/4 tsp sugar) better. That’s about 1 gram of carbs for one pound mushrooms. And that assumes all the sugar in solution goes into the mushrooms.
I’ve been cutting the mushrooms into quarters and using them on my cold meat (usually beef) lunches. They add a nice flavor.
If you do this, I recommend a sachet, that is, put the spices/herbs (save maybe the garlic) in a packet made of cheesecloth or similar. Otherwise, you can end up eating an allspice berry or a pepper berry by accident.