Cut the apples into pieces including the skins and seeds.
Gently mash the rowanberries with a potato masher or something similar.
Combine the apples, ginger, berries and water and bring to a simmer, then turn the heat to low and cook uncovered for 1 hour. Strain the liquid into a strainer lined with a double thickness of cheesecloth or a jelly bag and allow to drain until cool. Do not squeeze the liquid from the cheesecloth.
Discard the spent apple-berry mix.
Combine the reserved liquid (you should have 4 cups, if you don’t reduce the liquid until you have 4 cups, or adjust the sugar accordingly, using 1 cup sugar for each cup of reserved, strained liquid).
Doing set tests
Put some chilled plates, bowls, or other small containers in the freezer for doing set tests, preferably you testing bowls are made of metal as it conducts heat and cold quickly. Mix the liquid and sugar, then bring to a boil, turn the heat to medium, and cook until the temperature starts to hover around 220F. Begin doing set tests a little after the mixture hits 220F. 225F was about perfect for me, but let the set test be your guide.
You really want the jelly to boil hard for a bit, so adjust the position of the pan as needed to prevent it from overflowing. Cooking and reducing the jelly will take about 25-30 minutes, don’t be tempted to do set tests until around that time. Depending on the pan I’m using and how high the sides are, I may move it halfway off the burner to allow me to still use high heat without the pan overflowing. Cleaning up jelly overflows is, sticky and not fun.
For the set tests, pour a tablespoon or so into frozen metal bowls or spoons and allow to cool while the jelly simmers. Once you can see a gentle wrinkle on the top of the of the test surface when it cools you’re good, beat in the lemon juice, and pour the jelly into sterilized jars. Screw on lids and rings, and allow to cool. The jars will naturally seal themselves.