Rinse cranberries and pick through to remove any stems and soft, discolored, or wrinkled berries.
Instant Pot: Add syrup, orange zest, orange juice, and cranberries to pot. Stir to combine. Close lid and cook at high pressure for 1 minute, plus 7 minutes natural release. Then quick release remaining pressure and open lid carefully. Mash some of the berries with a spoon, potato masher, or immersion blender, if desired. Stir in 1/2 cup sugar. If needed, add 1 more tablespoon of sugar.
Slow Cooker or Stove Top: Add ingredients to a small slow cooker and cook on low for 4-6 hours. Or combine syrup, sugar, orange zest, orange juice to saucepan and bring to boil. Stir in cranberries and reduce heat to medium or medium-low to simmer. Stirring often, let simmer for about 10-15 minutes, until cranberries soften and burst.
Sauce will thicken as it cools!
You can double this recipe. Keep refrigerated for up to 14 days or freeze for up to 2 months.