Preheat the oven to 200°C, fan 180°C, gas 6. Grease a 23cm square baking tin.
Make the cornbread. In a large bowl, mix together the polenta, flour, baking powder, bicarbonate of soda, spring onions and Cheddar until combined. Make a well in the centre and set aside.
In a separate bowl, whisk together the eggs, yogurt, milk and melted butter. Pour this mixture into the well of the dry ingredients and stir until everything is combined. Pour the batter into the greased baking tin and bake in the oven for 20 mins until golden.
Meanwhile, make the chowder. Melt the butter in a large saucepan and gently cook the shallots and garlic for 3 mins until softened. Stir in the squash and potatoes, and cook for a further 5 mins. Pour over the vegetable stock, double cream and milk, and bring to a gentle simmer.
Add the sweetcorn and simmer for 5 mins. Using either a hand-held blender or a free-standing blender, purée half of the mixture until thickened and smooth. Return the puréed mixture to the pan, add the fish and parsley, and cook for a further 5 mins until the fish is opaque.
Serve the chowder with thick slices of the cornbread.