Melt butter in a large saucepan over medium-high heat. Stir in onion and garlic; cook until the onion has softened and the garlic has begun to turn brown, about 5 minutes. Stir in the celery and cook for 5 minutes. Add olive oil, Italian herbs, parsley, stock cubes and red wine; simmer for 5 minutes, stirring to break up the stock cubes. Stir in tinned tomatoes, sugar, cayenne and paprika. Bring to a boil, then reduce heat to medium-low and simmer for 10 minutes.
Take the sauce off of the heat and puree with a stick blender until smooth. Stir in the mixed seafood and return to a boil over medium-high heat. Stir in squid, reduce heat to medium-low and gently simmer for 30 minutes; season to taste with salt and pepper.
While the seafood sauce is simmering, prepare the polenta by bringing water to a boil in a saucepan over medium heat. Reduce heat to medium and slowly whisk in polenta with a wire whisk. Allow to simmer 5 minutes, whisking constantly. Stir in spinach, Parmesan cheese and mozzarella cheese. Reduce heat to medium-low and allow to slowly simmer 15 minutes, stirring frequently to keep from burning. Once the polenta has softened and turned creamy, remove from heat, season to taste with garlic salt and keep warm.
To prepare the vegetable mix, heat 2 tablespoons olive oil in a large frypan over medium-high heat. Stir in red onion and cook for 2 minutes. Add zucchini and cook for an additional 2 minutes, or until zucchini begins to brown. Stir in mushrooms and continue cooking until soft. Season with balsamic vinegar and salt and pepper to taste.
To serve, scoop a pile of polenta into the center of each plate; ladle the seafood sauce around, then top with vegetable mixture.