Slice the leek in half, all the way down, and fan it out under a running tap to get out any grit. Slice the white parts only into half-moons, discarding the green parts.
In a large frying pan over medium heat, heat a tablespoon of oil. Add the leeks to it, season them, and sautee for about 5 minutes.
Add the mushrooms, stir well to coat and cook down for another 5-10 minutes.
Add the lentils, then the spices, and stir well to combine.
Now add the tomato paste and the water, and allow to simmer until the mixture has thickened and the water has evaporated, about 10 minutes.
Remove from heat and place in the bowl of a food processor. Add the nuts and pulse to combine, until it resembles a grainy sausage texture. Taste and add seasoning. Set aside to cool.