Place heavy-bottomed, large pot or Dutch oven over medium heat. Add oil. If using butter, add to melt.
Then stir in onions and leek to soften, about 3 minutes.
Add celery and cook for 8-10 minutes, stirring to not burn.
Stir in garlic.
Add potatoes and stir.
Add enough stock to cover vegetables. If using frozen stock, no need to defrost.
Season with salt and black pepper.
Add fresh herbs. Place lid to cover pot and bring to a boil, then reduce heat and simmer until potatoes are tender but not mushy, about 10-15 minutes (depending on the size of the chop). Stir occasionally, adding a little more liquid if it has evaporated.
In a small bowl, use fork to dissolve cornstarch in water.
Remove lid to pot and stir in cornstarch mixture.
Immediately add fish and seafood. Submerge in liquid. Return lid to cover and once liquid begins simmering, allow seafood to cook for about 5 minutes or until opaque and tender. Do not over cook. The liquid will thicken as it simmers.
Add cream and stir until heated through but do not boil to avoid cream from separating. Remove herbs and adjust seasonings, adding more salt and/or pepper if necessary
Ladle hot chowder in bowls. Garnish with fresh parsley.
Serve hot with crusty bread.
In the video, I served it with a few pieces of freshly crispy battered fish.