Prepare the Clams: Rinse the clams thoroughly. In a large pot, bring 4 cups of water to a boil. Add the clams and cover. Steam until the clams open, about 5-7 minutes. Remove the clams and reserve the clam broth. Remove the clams from their shells and chop roughly. Set aside.
Cook the Bacon: In a large pot or Dutch oven, cook the diced bacon over medium heat until crispy. Remove the bacon with a slotted spoon and set aside, leaving the bacon fat in the pot.
Sauté the Vegetables: Add the chopped onion and garlic to the pot with the bacon fat. Sauté until the onions are translucent. Add the celery and carrots, and cook for another 5 minutes until they begin to soften.
Cook the Potatoes: Add the diced potatoes and the reserved clam broth to the pot. Bring to a simmer and cook until the potatoes are tender, about 15 minutes.
Combine Ingredients: Stir in the mashed potatoes, heavy cream, and whole milk. Add the chopped clams, fresh thyme, and the cooked bacon. Season with salt and pepper to taste. Simmer gently for another 10 minutes, stirring occasionally.
Garnish and Serve: In a separate pan, fry the whole belly clams until golden brown and crispy. Ladle the chowder into bowls, top each with a whole belly fried clam, and sprinkle with fresh parsley.
