In a medium saucepan, combine the lentils with 4 cups water. Simmer over medium heat until just tender, 20 to 30 minutes. Drain in a colander and let cool to room temperature.
Roughly chop the garlic, sprinkle it with a pinch of salt, and mash to a paste with the flat side of a chef’s knife. In a medium bowl, whisk the garlic paste, shallot, vinegar, ginger, mustard, fennel seed, 1½ tsp. salt, and ¼ tsp. pepper. Whisk in the oil.
In a large bowl, toss the lentils, radishes, fennel, fennel fronds, and chives with just enough of the vinaigrette to coat everything lightly (you may not need it all). Season to taste with salt and pepper. (You can prepare the salad to this point up to 4 hours ahead.) Just before serving, gently stir in the salmon and a few grinds of pepper. Garnish with chives and serve.