For the mirin creme fraiche: In a bowl, whisk the mirin into the creme fraiche until fully incorporated.
To serve: On 4 plates, spread an even layer of rock salt. Shuck the oysters, placing 3 on each plate. Put some rice wine mignonette on one oyster on each plate, the mirin creme fraiche on another and a dollop of harissa sauce on the third. Serve immediately.