Using a sharp paring knife or vegetable peeler, cut zest from lemon and in strips, avoiding white pith. Scrape any pith from zests, then combine zests and wine in a medium saucepan. Bring mixture to a boil, reduce to a simmer and cook for 1 minute, then add sugar, tea bag, vanilla, cinnamon, cloves and juniper berries. Simmer 5 minutes more, then remove from heat. Add apple and let steep, covered, 5 minutes. Strain mulled wine through a fine-mesh sieve. Serve hot in warmed heatproof glasses or cups.

Mulled wine can be kept overnight at room temperature. Reheat gently before serving.