Stir butter into rye flour and salt. Mix in water and vinegar and bring dough together. Form into a two discs and chill for 2 hours.
For the oyster stew base and the assembly:
Combine ingredients from cream to thyme in a small pot and hold just below a simmer for 1 hour. Strain, reserve, and let cool to room temperature. Mix in the reserved oyster liquor.
Heat the oven to 350° F. Roll one of the discs into a 11- to 12-inch circle, then transfer it to a 9-inch pie or tart plate. Dock the crust, line it with parchment paper and pie weight, then blind bake until just slightly browned, 12 to 15 minutes. Remove the pie weights and parchment and return to the oven for another 2 to 3 minutes, until the bottom is no longer visibly wet.
Let the pie crust cool. As it cools, roll out the other circle of dough.
Place the oysters in the cooled crust and add stew to cover. Place the remaining crust on top (it's okay if it sinks into the custard below) and crimp closed. Cut a decorative vent on top. Brush with egg wash and bake until golden brown, 35 to 40 minutes. The pie will be soupy and stewy inside, but the top crust should be cooked.