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1
Light a grill.
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2
Preheat the oven to 500.
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3
In a medium saucepan of boiling, salted water, cook the potatoes until tender, about 10 minutes.
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4
Drain and let cool.
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5
Peel the potatoes and halve them lengthwise.
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6
Tear off four 20-by-14-inch sheets of parchment or foil.
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7
Spread equal amounts of potatoes and mushrooms on half of each sheet.
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8
In a small bowl, mix the tamari with the vinegar and 1 tablespoon of olive oil.
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9
Drizzle over the potatoes and mushrooms and season with salt and pepper.
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10
Fold the sheets over the vegetables and crimp to seal, if using parchment; if using foil, fold in the edges to seal the packages.
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11
Transfer the papillotes to 2 baking sheets and bake for about 15 minutes, or until sizzling.
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12
Drizzle the bass fillets with 1 1/2 tablespoons of the olive oil, sprinkle with the rosemary and season with salt and pepper.
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13
Drizzle the plums with the remaining 1/2 tablespoon of olive oil.
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14
Grill the bass over high heat until nicely charred and just cooked, 3 to 4 minutes per side.
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15
Grill the plums, cut side down, until lightly charred, about 2 minutes per side.
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16
Transfer the bass fillets and plums to plates.
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17
Set a papillote on each plate and open it, pouring out the potatoes, mushrooms and any juices.
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18
Watch out for the hot steam.
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19
Serve garnished with watercress.
