Toast the fennel seeds in a skillet over medium heat until fragrant. Place the salt and toasted fennel seeds in a mortar and grind until they are well combined and the seeds have broken down. Pass through a very fine sieve and set aside.
Place the tomatoes, tequila, lime juice, and sugar into a blender and puree until smooth. Add the fennel salt, pepper and Tabasco sauce to taste.
Pour mixture into large ice cube trays and freeze. Just before serving remove from freezer, take the frozen cubes out of ice tray and place them back in the blender.
Puree until smooth but still frozen.
Spoon into chilled glasses, garnish with mint and serve immediately.