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1
Peel the casing from the cervelas. Cut the sausage into rounds 5 mm thick, or julienne into matchstick-sized batons for a more modern presentation. If warming the slices — which opens the surface to the vinaigrette — set a dry pan over moderate heat and warm them for 1 minute per side. The sausage should be supple and fragrant but not coloured.
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2
Combine the white wine vinegar, Dijon mustard, salt, and pepper in a small bowl. Whisk in the sunflower or grapeseed oil in a thin stream until the vinaigrette emulsifies into a uniform, opaque dressing. The surface should show no pooling oil.
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3
This is where the salad lives or dies: the vinaigrette must be assertive, not timid. Taste it before combining — it should read sharp and mustardy on the tongue, edged with acid, with the oil a background note only. Adjust vinegar if the balance tips towards blandness.
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4
Place the cervelas, paper-thin onion rings, and cornichon coins in a wide bowl. Pour the vinaigrette over and toss gently to coat every surface without breaking the sausage rounds. Set aside at room temperature for 20–30 minutes. The sausage is ready when its cut faces have dulled slightly, having absorbed the dressing.
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5
Shower the salad with a generous quantity of chopped flat-leaf parsley and snipped chives. Toss once more lightly. Serve at cool room temperature — the vinaigrette should smell bright and vinous, not flat, which refrigerator cold would produce.