Melt butter in a large stock pot over low heat. Continue cooking until lightly browned. Add flour into the butter and whisk to combine. Continue whisking and heating over low heat for about 5 minutes, or until the flour also starts to brown a bit. You want this mixture to be a nice nutty golden brown color.
Pour the clams & juice into the pot as well as the cream and half and half. Stir to combine and bring to a light simmer.
Put your diced potatoes on a plate and microwave for 3-5 minutes, checking frequently. You want the potatoes to be about 1/2 way to fork tender. Remove from microwave and add to the stock pot.
Add the thyme and white pepper, stir, and simmer for about 10-15 minutes, or until the potatoes are cooked.