Place raspberries in a food processor; cover and process until blended. Strain raspberries, reserving juice. Discard seeds. Place juice in a Dutch oven; stir in sugar and lemon juice. Heat over medium-high heat to 190°. Do not boil.
Remove from heat; skim off foam. Carefully ladle hot mixture into five hot 1-pint jars, leaving 1/4-in. headspace. Wipe rims; screw on bands until fingertip tight.
Place jars into canner simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 10 minutes. Remove jars and cool.
To use concentrate: Mix 1 pint concentrate with 1 pint tonic water. Serve over ice.
In a large saucepan, combine the cream, salt, and a 1/2 cup of the sugar. Cook over medium-low heat, stirring gently, until the cream just starts to steam. Off the heat, stir in the lemon verbena sprigs, and steep for 10 or more minutes.
In the meantime, separate out your egg yolks (reserve the whites for omelets or meringue) and, in a medium-sized bowl, whisk with the remaining 1/4 cup of sugar. Set aside.
Remove the lemon verbena sprigs from the cream mixture, and reheat over medium-low heat until everything is starting to steam and small bubbles form along the edge. Whisk a quarter cup of cream into the yolks, and then add several more scoops of warm cream to temper the yolks before you add then to the cream. When the yolks are warm, slowly whisk them into the hot cream mixture. Continue cooking the cream mixture over low heat for five minutes, or until it thickens enough to coat the back of a wooden spoon. If your egg tempering resulted in any chunks, or if you’re an ice cream purist, you can strain the mixture through a mesh sieve. (I am a lazy sort, and wouldn’t think of dirtying my strainer unless I absolutely had to.)
Remove from heat, stir in the buttermilk, and chill in the fridge for three hours. If you’re in a hurry, set the bowl of custard over an ice bath and it will chill in an hour.
Process ice cream according to your ice cream maker’s instructions, spoon into storage containers (I use covered glass loaf dishes) and then cure in the back of the freezer for at least three hours. Serve with an herb garnish, or maybe a slice of fruit pie.