Preheat the oven to 180C/350F/Gas 4.
For the cornbread, mix together the flour, polenta, baking powder, lemon zest and cayenne pepper in a bowl until well combined. Gradually add the water in a thin stream, stirring well until the mixture comes together as a thick batter (you may not need to use all of the water). Stir in the lemon juice and garlic and season, to taste, with salt and freshly ground black pepper.
Heat half of the oil in a 20-25cm/8-10in ovenproof frying pan over a medium to high heat. When the oil is smoking, pour in the cornbread batter. Fry for 4-5 minutes on each side, or until beginning to set.
Drizzle the remaining oil over the cornbread, then transfer the pan to the oven and continue to cook for a further 4-5 minutes, or until golden-brown on top and cooked through. Remove from the pan and set aside to cool slightly.
Meanwhile, for the topping, heat the oil in a frying pan over a medium heat. Add the onion, celery and garlic and fry for 4-5 minutes, or until softened. Stir in the spices and herbs and cook for a further 1-2 minutes, or until fragrant.
Add the chopped tomatoes and stock and bring to the boil. Reduce the heat until the mixture is simmering, then continue to simmer for 5-8 minutes, or until thickened.
Add the kidney beans and butter beans and cook for a further 3-4 minutes, or until heated through.
To serve, cut the cornbread into six equal-sized pieces. Place one into the centre of each of six serving plates. Spoon over the bean mixture.
