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1
In a large saucepan, cover the potatoes with water and add a large pinch of salt, bring to a boil and simmer over moderately high heat until the potatoes are tender, about 20 minutes.
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2
Drain and let cool, then slice the potatoes 1/4 inch thick and transfer to a bowl.
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3
Heat the olive oil in a large pot.
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4
Add the garlic, thyme sprigs and onion and cook over moderately low heat until softened, about 12 minutes.
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5
Add the rose and 2 tablespoons of the lemon juice and bring to a boil over high heat.
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6
Add the mussels, cover and cook, shaking the pan a few times, until the mussels open, about 7 minutes.
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7
Set a colander over a large bowl.
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8
Pour the mussels into the colander and when cool enough to handle, shell them.
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9
Add the mussels to the potatoes and cover with plastic wrap.
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10
Slowly pour the mussel juices back into the pot, stopping before you reach the grit at the bottom.
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11
Bring to a boil over high heat and cook until reduced to 1/3 cup, about 12 minutes.
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12
Let cool.
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13
In a medium bowl, whisk the mayonnaise with the remaining 3 tablespoons of lemon juice and the mustard.
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14
Stir in the capers, cornichons, parsley, chives and the reduced mussel juices.
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15
Season with salt and pepper.
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16
Fold into the potatoes and mussels.
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17
Serve the salad at room temperature or slightly chilled.
