Place the water in a large pot and bring to a boil. Add the lemon slices and boil for 20 minutes. Strain, reserving the water.
Place the jalapeños in a blender and add the oil. Puree to make a thick paste.
Add the lemon slices, a few at a time, along with 3 cups of the reserved water, a half cup at a time. You may have to do this in batches in you don’t have a large blender. Puree to a thick sauce. Pour into bottles and label.