Preheat the grill to its highest setting.
Melt the butter in a frying pan and add the oil.
Fry the leek and garlic for about five minutes, or until soft.
Add the peas, tarragon and lemon juice. Stir in the crème fraîche along with half of the gorgonzola and half of the walnuts until warmed through.
Add the cooked penne and spoon into four dishes.
Top the dishes with the rest of the nuts and cheese and place under the grill for three minutes, or until golden-brown.
Serve with garlic bread.
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