Put the lentils in a pot with the water and bring to a gentle boil over medium-high heat. Reduce the heat to a simmer and cook, uncovered, until the lentils are tender, 15 to 20 minutes, adding water if the liquid has evaporated and the lentils are still tough. Drain the lentils and set aside to cool.
Using a vegetable peeler, peel the carrots. Rest the bottom of one of the peeled carrots on a cutting board. Starting at the skinny tip of the carrot, press firmly down the length of the carrot with the vegetable peeler to create thin shavings. Repeat with the remaining carrots. (If you prefer, you can use a mandoline instead.) Add the shaved carrots to a mixing bowl along with the broccoli sprouts and onion.
To make the dressing, in a separate small bowl, whisk together the tahini, olive oil, agave nectar, lemon juice, salt, and pepper. Add the water, 1 tablespoon at a time, until you reach the consistency of a basic vinaigrette.
Add the dressing to the bowl with the vegetables and toss to coat. Add the lentils and half of the pistachios and toss again. Sprinkle the remaining pistachios on top and serve immediately.