Shake all ingredients (including the grapefruit swath) with ice for 15 seconds, strain into a chilled coupe or Nick & Nora glass. No garnish.
This is definitely for those who like (really like) the earthy bitterness of Suze. If you want something a bit less intense, do 1 oz pamplemousse and .5 oz Suze.
If you don't have Malfy con Limone, use a low proof, citrusy gin such as Plymouth, Aviation, etc.