Add an inch of water to the bottom half of a double boiler and bring it to a simmer over a medium flame. (See below if you do not have a double boiler.).
Combine the milk and cream in another pot and heat the mixture gently over a low flame.
In the top of the double boiler, whisk the egg yolks and sugar to combine. Set the mixture over the simmering water and keep whisking as you slowly pour in the warm milk and cream. Continue to whisk until the mixture begins to thicken—do not allow it to boil or it will curdle. When the mixture has thickened enough to lightly coat the back of a spoon, remove from the heat.
Stir in the grated lemon zest and let cool to room temperature.
Stir in one tablespoon of rosewater and taste. If a stronger flavor is desired, add a little more rosewater, a teaspoon at a time. Pour the mixture into a covered container and chill in the refrigerator for 3 or more hours.
Taste the mixture after it is cold and adjust the flavor if necessary. Freeze in an ice cream maker according to the manufacturer’s directions. For my ice cream maker, the mixture was done churning in about 30 minutes.
Serve as soon as it is ready, garnished with organic, unsprayed rose petals if desired. Or place in a covered container and store in the coldest part of your freezer.
Note: If you do not have a double boiler, you can approximate one by setting a large, heat-proof bowl into the top of medium, heavy-bottomed pot filled with an inch of water. Be sure that the simmering water does not touch the bottom of the bowl, or the ice cream mixture may curdle. The bowl should sit partway into the pot, but well above the water. <<<--this technique works out well for me.