For the falafel crust, combine the chickpeas, potato, parsley, leek and/or green onions and butter in a food processor and pulse until the mixture is finely chopped and well mixed. Add the bread crumbs, coriander and cumin and pulse to mix. Add salt and pepper to taste. Transfer to a bowl and set aside.
For the hummus, wipe out the food processor bowl. Add the chickpeas, tahini, lemon juice, olive oil and garlic. Process until smooth, then season to taste with salt and pepper. Transfer the hummus to a bowl, cover and chill until needed.
Combine the vinaigrette ingredients in a large bowl and whisk to mix. Season to taste with salt and pepper. Add the cucumber and tomato and toss well. Chill until needed.
Preheat the broiler and set the oven rack 4 to 6 inches below the element. Season the salmon fillet pieces with salt and pepper. Heat the oil in a heavy ovenproof skillet, preferably nonstick. When hot, add the salmon and sear until browned, 1 to 2 minutes per side. Spread the top of each fillet piece with some of the falafel crust mixture, transfer the skillet to the oven and broil the salmon until the crust is bubbly and the salmon is cooked to taste, about 2 minutes for medium or 3 to 4 minutes for well done. (If the crust is browned before the salmon is cooked, turn off the broiler, close the oven door and let the salmon sit for a minute or two to continue cooking.).
To serve, spoon some of the hummus onto the center of individual plates and spoon the tomato and cucumber salad around. Set the salmon pieces on the hummus and serve right away.