Combine all marinade ingredients in a non-reactive pan or plastic storage bag. Add the venison loin blocks and gently massage the marinade into the meat. Refrigerate overnight. The next day, blot the loin dry, lightly brush with olive oil and season with salt and pepper (use half as much salt as you normally would as the koji is seasoned). Heat a stainless steel or cast iron skillet to medium-high and sear the loin on all sides. Transfer the seared loin to a baking dish and finish in an oven preheated to 350 degrees. When the loin reaches an internal temperature of 130 degrees, remove from the oven, chill and reserve. Deglaze and film the skillet over medium-high heat. Thinly slice the porcini mushrooms, season both sides with salt and pepper and quickly sear on one side only (they should just color). Remove to a cool plate and reserve.
For the dashi barley, add olive oil, ginger and barley to a heavy-bottom sauce pot and toast over medium-high heat until fragrant. Carefully add dashi and bring to a simmer. Cook until tender (approximately 45-50 minutes). Season and reserve.
To make the vinaigrette, combine all ingredients except for the oils in a non-reactive bowl. While whisking, slowly add the oils to emulsify. Adjust seasoning and reserve.
To plate, combine purslane, pickled prunes, figs, shallots, black walnuts, shiso leaves and oil in a non-reactive bowl and gently toss to combine. Season with the vinaigrette and toss once more. Spoon the salads equally onto six plates and top with a slice of venison. Coat the venison with a teaspoon of the vinaigrette and garnish with fleur de sel and a sprig of tarragon.