2 tablespoons finely chopped mint and thinly sliced red onion, for garnish
In a large pot, cover the lentils with 2 inches of water, bring to a boil and add 1/2 teaspoon of salt. Simmer the lentils over moderate heat until tender, 25 to 30 minutes. Drain well and transfer to a large bowl.
In a small saucepan, cook the butter over moderately low heat until golden brown, 3 to 4 minutes. Remove the pan from the heat and carefully add the vinegar. When the foam subsides, swirl in the cumin. Stir the brown butter vinaigrette into the lentils. Add the pistachios and feta, season with salt and pepper and toss gently. Garnish with the mint and red onion and serve warm.