Bring smashed garlic, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and 3 cups water to a simmer in a medium
saucepan over medium heat. Add mussels and cook, uncovered, until opened, about 3 minutes. As mussels open, use tongs to transfer them to a medium bowl; discard any mussels that remain closed after 5 minutes or so. Chill mussels 15 minutes.