Taste the mixture to make sure there is enough salt, then transfer to a blender or food processor. Pulse until the eggplant and the mushroom are finely chopped but still a little chunky. Spoon the mixture into a large bowl and stir in half a cup of breadcrumbs. Check the mixture by scooping a heaping spoonful (about 3 tablespoons) and forming into a ball. If the mixture is tacky and doesn’t form a round ball, add more breadcrumbs. The mixture will be sticky, but should still hold the shape of a ball. Form the rest of your meatballs and place on a plate.