In a large saucepan, melt the margarine over medium heat. Add the chopped onion and sauté until it turns soft and fragrant, about 3–4 minutes.

Stir in the celery, carrots, corn, cannellini beans, and water. Add the bay leaf, thyme, basil, garlic powder, and black pepper. Raise the heat and bring the mixture to a boil, then reduce to a gentle simmer. Cover and cook for around 10 minutes, or until the vegetables are tender.

While the chowder simmers, whisk together the flour and 1/2 cup of the milk in a small bowl until the mixture is completely smooth.

Pour the remaining 1 1/2 cups of milk into the soup pot and stir well. Gradually whisk in the flour mixture, ensuring it's fully incorporated and lump-free. Let the chowder cook for a few more minutes, stirring occasionally, until slightly thickened.

Gently fold in the drained tuna and half of the chopped parsley. Heat through, then remove from the heat.

Ladle the chowder into bowls and garnish each serving with the remaining parsley. Serve warm.