Set a sous vide cooker inside a large pot and pour water to the fill line to make the water bath. Preheat sous vide to 185 degrees F (85 degrees C).
Wrap rosemary and garlic loosely in cellophane to keep it separate from the asparagus. Place in a resealable bag with asparagus and butter. Remove air from bag and seal. Set sous vide timer for 15 minutes.
Remove cooked asparagus from the bag, place on a plate, and discard the rosemary and garlic.