Wipe the sardines with newspaper—no need to scale them. Remove the heads, gut them, and butterfly open. Carefully remove the backbone to lift the fillets. Rinse under cold water and pat dry thoroughly with paper towels. Layer in a terrine, seasoning with salt and pepper.
Peel and dice the carrot and onion. Sauté gently in 1 tablespoon oil without browning. Add the remaining oil, vinegar, peeled and chopped garlic, cayenne, and 1 sprig tarragon. Simmer for 3 minutes, then pour hot over the sardines. Cool, cover, and refrigerate for 24 hours.
The next day, boil the potatoes in salted water for about 20 minutes until tender. Drain, peel, and slice thinly. While warm, dress with 2 tablespoons vinaigrette.
Pick and wash the parsley, chervil, mint, burnet, and hyssop leaves. Wash the arugula and chives, then coarsely chop everything. Toss with 1 tablespoon vinaigrette.
Arrange potato slices on plates, top with marinated sardine fillets, and add the herb salad to one side. Finish with crushed pink peppercorns and remaining tarragon leaves. Serve immediately.
