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Meanwhile, melt half the margarine, and slice the onion, cook gently till tender not brown, and add to lentils with lemon juice and rind, marmite, salt and pepper.
Liquidise the mixture using a food processor. Saute chopped celery in rest of margarine till tender, place in base of dish, pour on the lentil mixture, and top with breadcrumbs. Bake till golden brown about 45 minutes at Gas 4/350F/180C.