In a large pot combine the sugar, water, lemon slices zest and a pinch of salt. Boil until the sugar has dissolved. Add about half of the lemon juice and continue to boil for another 30 minutes to reduce.
Strain the liquid through a fine mesh sieve. Reserve the boiled rinds and zest in a small bowl - it will begin to crystallize as it sets. Chill the strained liquid until very cold, at least 2-4 hours.
Stir the rest of the lemon juice into the chilled liquid. Use an immersion blender or a standing blender to combine in the Greek yogurt (it will not mix in naturally unless blened). Stir in the lingonberry jam by hand until the individual berries are susended in the liquid.
FREEZING METHOD #1 (smoother results): Pour the liquid into an ice cream machine. Follow the ice cream machine instructions to blend until frozen. Transfer the frozen ice cream to a large, airtight storage container. Freeze until solid - at least 8-10 hours.
FREEZING METHOD #2 (icy results): Pour the liquid into a large, airtight container. Freeze until solid - at least 8-10 hours.
Enjoy while studying for your O.W.Ls!