How to Make It
In a small bowl, whisk the vinegar with the chopped tarragon, mustard, honey and ¼ cup of the olive oil. Season the vinaigrette with salt and pepper.
In a large saucepan, heat 1 tablespoon of the olive oil. Add the chard stems, shallot and garlic and cook over moderate heat, stirring occasionally, until the stems are softened, about 5 minutes. Stir in the chard leaves in large handfuls and cook until just wilted, about 5 minutes. Stir in half of the vinaigrette and season with salt and pepper.
Meanwhile, season the salmon with salt and pepper. In a large nonstick skillet, heat the remaining 1 tablespoon of olive oil until shimmering. Add the salmon, skin side down, and cook over moderately high heat, pressing gently with a spatula to flatten, until the skin is browned and crisp, about 3 minutes. Flip the salmon and cook until medium within, about 3 minutes. Transfer the salmon and wilted chard to plates. Garnish with tarragon leaves and serve, passing the remaining vinaigrette at the table.