Bring 1 cup of the cream and the milk to a simmer in a heavy medium saucepan.
Puree the cucumber in a blender or food processor (or juicer if available), and then strain the juice through a fine sieve until you have 3/4 cup cucumber juice.
Combine the sugar, cilantro and mint in a food processor and process until the herbs are finely ground.
Whisk the yolks with the herb sugar in a heatproof bowl until combined. Gradually whisk in the hot cream mixture until blended, tempering the egg yolks. Return the mixture to the saucepan and add the salt. Stir over medium-low heat (do not boil) until the custard thickens slightly and coats the back of a spoon or an instant-read thermometer inserted into the mixture registers 170 degrees F, about 3 minutes.
Pour the custard through a strainer into a medium bowl set over a slightly larger bowl filled halfway with equal parts ice and cold water.
Stir the mixture until the custard is cool to the touch, about 5 minutes. Remove the bowl from the ice water and whisk in the cucumber juice and yogurt. Refrigerate until well chilled, and then transfer to the ice-cream maker and process according to manufacturer's instructions. Cover and freeze in a freezer container until firm. Serve with the Indian Pistachio Brittle.
For Indian Pistachio Brittle:
Line a large sheet tray with parchment paper.
Toast the coriander, cumin and fennel seed in a small heavy skillet over medium heat, stirring until fragrant, about 4 minutes, and then transfer to a plate.
Heat the sugar, honey and salt in a 2-quart heavy saucepan over medium heat, stirring often, until deep golden, about 5 minutes. Stir in the pistachios and spices and remove from the heat. Pour the mixture onto the parchment, smoothing the top and spreading thinly with a metal offset spatula, and let cool to room temperature.
Break the brittle into pieces using the bottom of a small heavy saucepan, and then crush into coarse bits. Transfer to an airtight container.
NotesThe ice cream can be made up to 3 days before serving.