Sauté the aromatics. Melt the butter in a large, heavy-bottomed pot or Dutch oven over medium heat. Add the diced onion and celery, and cook, stirring occasionally, until the onion becomes translucent and soft — about 5 minutes. You'll know it's ready when the kitchen smells sweet and the onion has lost its raw, sharp scent. Add the garlic and cook for another 30 seconds, until fragrant.

Make the roux. Sprinkle the flour over the softened vegetables and stir constantly for 1-2 minutes. The mixture will look pasty and clumpy at first, but keep stirring. You'll notice it turning a pale golden colour and smelling slightly nutty. This step is crucial for thickening the chowder without lumps.

Add the liquid. Slowly pour in the chicken stock, whisking continuously to prevent lumps from forming. The mixture will start to bubble and thicken almost immediately. Add the reserved clam juice, the bay leaf, and the dried thyme. Stir well.

Simmer the potatoes. Add the diced potatoes to the pot. Bring the liquid to a gentle boil — you'll see small bubbles breaking the surface — then reduce the heat to low. Cover and let it simmer for 12-15 minutes. The potatoes are done when you can easily pierce a cube with a fork; the tip should slide in with no resistance.

Add the cream and clams. Pour in the double cream and stir gently. Add the drained clams. Increase the heat to medium and cook for 2-3 minutes, just until the chowder is heated through. Do not let it boil at this stage, or the cream might curdle. You'll see the colour shift to a creamy, pale ivory.

Season and serve. Remove the bay leaf and discard. Taste the chowder and season generously with salt and pepper. Ladle into bowls and garnish with fresh parsley. Serve immediately with crusty bread or oyster crackers.