Place the haddock fillets and seafood stock in a large pot. Bring to a gentle simmer over medium heat.
Poach the fish for 8 to 10 minutes. The haddock is safe to eat when the internal temperature reaches 145°F (63°C) on a meat thermometer.
Use a slotted spoon to transfer the cooked fish to a plate. Pour the cooking liquid into a heat-safe bowl and set it aside.
Wipe the pot clean and place it back on the stove over medium heat. Add the chopped bacon and cook until crisp, about 5 to 7 minutes.
Remove the bacon pieces and leave the fat in the pot. Add the butter and diced onions, cooking for 5 minutes until soft.
Stir in the diced potatoes, thyme, and the reserved stock. Bring to a boil, then lower the heat and simmer for 10 to 15 minutes until the potatoes are tender.
Reduce the heat to medium-low. Pour in the whole milk and heavy cream, stirring gently to combine.
Break the cooked haddock into large chunks and add it to the pot. Heat for 5 minutes without letting the soup boil.
Season with salt and black pepper. Serve warm, topped with the crispy bacon and fresh parsley.