Preheat a large stockpot or Dutch oven over medium heat. Melt butter and add onion, carrots and celery. Season with 1/2 teaspoon salt and a few grinds of black pepper, to taste. Cook, stirring occasionally, until soft and onions are translucent, about 5-7 minutes. Add garlic and thyme leaves and cook for 1 minute.
Stir in flour and cook for another minute. Gradually whisk in chicken broth and milk, and cook, stirring constantly, until liquid comes to a boil and is slightly thickened, about 1-2 minutes. Add potatoes and bring back to a boil, then reduce heat and simmer until potatoes are tender, about 12-15 minutes.
Stir in cream cheese until melted and add turkey, chard and corn. Cook until turkey is warm and chard is wilted, about 10 minutes. Season with salt and pepper, to taste. If the chowder is too thick, add more milk as needed until desired consistency is reached.