Using a steamer, steam the carrots for 8-10minutes or until they’re cooked through. When they’re cool enough to handle, add them to a food processor with the remaining ingredients for the purée and blend until it reaches a smooth consistency. Adjust the seasoning, adding more salt if necessary. If the mixture seems too thick and you’re having trouble mixing it all together, add up to 6 table spoons of the water you used for steaming the carrots.
In the mean time, bring a medium sized pan filled with 4 cups of water to a boil. Add the lentils, reduce the heat to medium and cook for approximately 12-14minutes, or until they’re cooked through but still have some bite. Add the salt, drain the lentils and rinse under cold water. Transfer the lentils to a bowl and mix in the chopped parsley.
Caramelized onions: Heat a large non stick skillet over medium heat and add the olive oil, onions and salt. Cook, stirring often, for about 15 to 20 minutes, or until the onions are really soft. In the last minutes of cooking, turn the heat up to high and cook for additional 3 to 4 minutes, or until they crisp up a little.
Hazelnut mustard dressing: In a skillet over medium-high heat, toast the hazelnuts until they’re golden brown (4 to 5 minutes). When they’re still hot, transfer to a clean kitchen towel and rub them against it to remove as much of its skin as possible. Transfer the hazelnuts to a food processor and process until smooth with the remaining dressing ingredients.
To serve, put a few tablespoons of carrot purée on 4 different plates, topping with the lentils, followed by the caramelized onions and a gentle drizzle of the hazelnut dressing. Add a few arugula leaves, if desired, and serve.