Heat the oil and butter together in a pan and add the onions. Fry the onion for 10-12 minutes on a low heat, until it starts to caramelize and turn lightly golden. Meanwhile, grill the sausages for 12-15 minutes, turning regularly until cooked through. Stir the mustard, lentils, crème fraîche and parsley into the lentils. Season to taste.
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