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1
Rub quinces with a damp paper towel to remove fuzz. Cut 4 quinces into large pieces (no need to peel, core, or remove seeds); set remaining 2 quinces aside. Place quince pieces in a large saucepan and add cold water to cover. Bring to a boil, reduce heat, and simmer, adding more water as needed to keep covered, until quinces are very soft, 60-75 minutes. Straincooking liquid into a large bowl; discard solids. Wipe out saucepan and reserve.
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2
Meanwhile, peel reserved quinces and cut flesh off of cores into 4 lobes (be careful; core is very hard). Place in reserved saucepan and add tea bag, lemon zest, sugar, and quince cooking liquid. Scrape in vanilla seeds; add pod. Bring to a boil, then reduce heat and gently simmer, stirring occasionally, until quinces are a deep pinkish-red and tender and liquid is syrupy, 45-60 minutes. Let quinces cool in syrup, then stir in lemon juice. Remove pod.
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3
Quinces can be poached 5 days ahead. Cover and chill.
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4
Place yogurt in a medium bowl and gradually add heavy cream, whisking until smooth.
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5
Bring milk, sugar, and salt to a simmer in a small saucepan over medium-low heat, stirring to dissolve sugar. Add tea bags and scrape in vanilla seeds; add pod. Remove from heat and let sit until flavors are infused, 15-20 minutes.
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6
Meanwhile, sprinkle gelatin over 1/4 cup cold water in a small bowl; let sit 10 minutes to soften.
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7
Reheat milk mixture over medium-low just until steaming. Remove from heat and add gelatin, stirring to dissolve. Strain into bowl with yogurt mixture; discard solids. Whisk until smooth. Pour into glasses or ramekins, dividing evenly. Chill panna cotta until set, at least 4 hours. Serve topped with poached quinces and syrup.
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8
Panna cotta can be made 2 days ahead. Cover and keep chilled.
