1.Cook lentils in a medium saucepan of boiling water, uncovered, for 12 minutes or until just tender; drain.
2.Meanwhile, heat oil in a large deep frying pan over medium-high heat. Cook onion, garlic and seeds, stirring, for 5 minutes or until onion is soft. Add carrot, mushrooms and tomatoes; cook covered, for 10 minutes, or until carrot softens. Stir in paste; cook for 1 minute. Add in stock; bring to the boil. Cook, uncovered, for 3 minutes or until thickened slightly. Stir in lentils and spinach; season to taste. Cover to keep warm.
3.Make polenta topping: Bring stock to the boil in a large saucepan. Gradually add polenta and thyme to stock, stirring constantly. Reduce heat; cook, stirring, for 10 minutes or until polenta thickens. Stir in oil and yeast. Season to taste.
4.Preheat grill (broiler) on high. Place lentil mixture in a 2-litre (8-cup) ovenproof dish; spread with polenta topping. Grill for 15 minutes or until golden and crisp.
5.Serve topped with extra thyme. Season to taste.
French-style green lentils, grown in Australia, are related to the famous French lentils du puy; these green-blue, tiny lentils have a nutty, earthy flavour and a hardy nature that allows them to be rapidly cooked without disintegrating.