Add the passion fruit, mezcal, agave, and orange juice to a blender and blend until creamy and smooth. Taste and adjust sweetness / strength of alcohol by adding more of either as desired.
To rim serving glasses, add ~1 Tbsp salt to a small plate. Rim glasses lightly with juice by running leftover orange halves (or a lime wedge) around the rim of a glass, then immediately flip and dip the glass in salt.
Divide between glasses and enjoy! Leftovers keep for 1-2 days, but will not be “slushy.” Can also freeze into ice cubes to reblend later.