Wash the orange and lemon thoroughly. Slice the orange into 1/4-inch (6 mm) rounds. Slice lemon the same way if using. Set aside.
Gather cinnamon sticks, cloves, allspice berries, star anise, and peppercorns if using. Optionally place all whole spices into a reusable spice bag or cheesecloth for easy removal.
Pour the apple cider into your large pot or Dutch oven. Add the sliced orange rounds (and lemon, if using), whole spices or spice bag, and grated nutmeg. Stir gently to combine.
Place the pot over medium-low heat. Bring to a gentle simmer, not a boil (target 160-165°F / 71-74°C). Heat slowly for about 10 minutes until steaming and fragrant.
Add the sweetener starting with 2 tablespoons, stir to dissolve, and taste. Adjust with remaining tablespoon if desired, keeping in mind sweetness will concentrate as it simmers.
Continue simmering on low heat for an additional 15 to 20 minutes, stirring occasionally. Do not exceed 45 minutes total simmering time to prevent bitterness.
If making an alcoholic version, remove from heat and stir in 1 to 2 cups of bourbon, spiced rum, or brandy. Return to lowest heat to keep warm, but do not bring back to a simmer.
Remove from heat. If using a spice bag, lift it out and discard. Otherwise, strain the cider through a fine-mesh strainer into a bowl or pitcher. Ladle into mugs and garnish as desired. Serve immediately.