Drain the oil from the tuna, reserving the oil. Place the tuna in a mixing bowl and flake it with a fork. Add the celery, onion, cornichons, lemon juice, mayonnaise, 2 tablespoons of the reserved oil, the mustard, salt, and pepper and mix well. Cover and refrigerate for a few hours to allow the flavors to develop.
Toast the bread and spread each slice with a layer of hummus. Spread the tuna salad on each piece of bread, garnish with slices of radish, and serve immediately.
For the Hummus:
Place the chickpeas, 2 tablespoons of the reserved liquid, the tahini, garlic, lemon juice, hot sauce, and salt in the bowl of a food processor fitted with the steel blade and process until coarsely pureed. The hummus should be moist and thick; add more lemon juice or reserved chickpea liquid to thin, if necessary. Cover and refrigerate for several hours for the flavors to blend. Taste for seasonings. Yield: 2 cups.
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