In a large bowl whisk together the dijon mustard and vinegar together until well incorporated. Slowly add the oil in a steady stream, whisking constantly. A thick creamy, fully emulsified dressing is your goal. Add a pinch each salt and pepper.
Place the salmon cubes into the large bowl with the mustard vinaigrette. Toss to coat; cover and refrigerate 4 to 8 hours, tossing them once in a while to assure even marination.
Just before serving. Slice the avocados in half and remove the pits. Use a melon baller to makes as many nicely formed balls as possible, placing them in a medium bowl as you work. Pour the lemon juice over them to keep them from discoloring. Toss gently to combine.
Plate each salad individually by dividing the arugula and cilantro evenly between each plate. Drain the salmon and avocado, discarding the liquid. Divide the salmon and avocado between the plates. The liquid that clings to them should be enough of a dressing. But a plain vinaigrette could be added if you wish. Garnish each with whole mustards seeds and minced chives. Season to taste with more salt and pepper if you like.
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