Sauté the aromatics. Heat olive oil and butter in a large pot over medium heat. Add onion, celery, and carrot with a pinch of salt.
Cook 5–6 minutes, stirring, until softened but not browned. Stir in garlic for 30 seconds until fragrant.
Build the base. Add bay leaf, smoked paprika, and thyme. Stir for 30 seconds to bloom the spices.
Add liquids and potatoes. Pour in the stock and add the diced potato.
Bring to a gentle simmer, then reduce heat to maintain a low simmer. Cook 10–12 minutes, until potatoes are just tender.
Add milk and corn. Stir in the milk and corn. Keep the heat at a gentle simmer; avoid boiling to prevent curdling and keep the chowder silky.
Poach the fish. Season the fish cubes with a little salt and pepper.
Nestle them into the pot in an even layer. Simmer on low 5–7 minutes, or until the fish is opaque and flakes easily. Do not stir vigorously—a few gentle nudges are enough to keep the pieces intact.
Finish with herbs and lemon. Remove the bay leaf. Stir in parsley, dill, lemon zest, and lemon juice.
Taste and adjust salt and pepper. For extra creaminess, whisk the Greek yogurt in a small bowl with a ladle of hot chowder, then stir it back into the pot off the heat.
Serve. Ladle into bowls and top with a sprinkle of fresh herbs and black pepper. Add a squeeze more lemon if you like.