Make the cream mixture. Pour the cream into a medium-sized saucepan. Use a paring knife to slice open the vanilla pod, lengthwise. With the back of the knife, scrape the vanilla beans directly into the saucepan with the cream, and add the emptied pod as well. Add the lemon zest and lavender to the cream and place the pan over low-medium heat. Gently whisk every so often until the mixture just begins to simmer, about 3 minutes. Cover and let the flavors steep for about 30 minutes. Then strain the mixture over a bowl and discard the zest, lavender and vanilla pod. Pour the mixture back into the saucepan. (At this point it should be only slightly warm or close to room temperature.)
Mix the egg yolks with the sugar. Add the sugar to the egg yolks in a medium-sized mixing bowl, and beat with an electric mixer or by hand with a whisk. Beat until they're slightly thick and pale yellow, about 1 minute on high with the mixer, or a few minutes by hand.
Combine the egg mixture with the cream mixture. Add lemon juice to the cream mixture in the pan and whisk until smooth. (Do not add the lemon juice if the cream mixture is still hot! It should be warm to room temperature at this point.) Add about half of it into the eggs and whisk to blend. Now pour this into the pan with the remaining cream mixture.
Gently cook while whisking. Turn the heat to low-medium and cook, whisking slowly, almost constantly until it's thick and coats the back of a spoon, about 8 minutes. Remove the pan from the heat just as it begins to simmer, but isn't quite simmering. Do not let it boil. You have now made custard! Immediately pour the custard into a large bowl.
Refrigerate. Gently press plastic wrap directly on top of the custard so the entire surface is covered. (This prevents it from forming a "skin.") Chill in the refrigerator overnight.
Mix in ice cream machine. When you're ready, freeze the custard according to your ice cream machine's directions.
Freeze. When it's done, spoon the frozen custard into an air tight container and place it in freezer until you're ready to serve.