Place potato in a medium saucepan. Add enough water to cover. Bring to boiling; reduce heat. Cover and simmer for 15 minutes. Add thawed spinach. Cook 5 minutes more or until potato is tender.
Drain mixture, reserving cooking liquid. Cool slightly. Puree in a food processor or blender until smooth, adding enough of the reserved cooking liquid until of desired consistency (about 1/4 cup). Use within 1 day, or freeze for up to 1 month.
Spoon finished food into ice cube trays. Cover with waxed paper and freeze until firm. When ice cubes are frozen, remove cubes from tray and store in a resealable plastic freezer bag or freezer container. Label package with contents and date. Use within 1 month. Before serving, let thaw in the refrigerator overnight.
Heat thawed mixture to lukewarm in a small dish set in a pan of hot water. Stir before serving. Always sample a small bite to make sure the food is not too hot.
