Slice off the root end of the leek and the dark green top. Slice the leek in half lengthwise. Place both halves cut side down and cut them into thin half-moon pieces. Since leeks have lots of sand and dirt between the layers, place the slices in a bowl of water. The sand will settle to the bottom. Scoop the leeks out of the bowl with your hands so the sand stays on the bottom of the bowl.
Cut the skin off the pumpkin; then cut into coarse chunks and rinse them. If using canned pumpkin, skip this step.
Peel, rinse, and cut the potatoes into cubes.
In a stock pot, heat the butter and brown the leek pieces and thyme together.
Add the squash and the potatoes and cover with water. Season with salt and pepper. Cover with a lid and cook for about 20 minutes.
Remove the thyme; if you want more fiber, add some oat bran and blend with an immersion blender.
If it is too liquid, you may boil it for a few minutes more. If it is too thick, add some hot water.
Garnish your soup with a little extra virgin olive oil, some fresh thyme leaves, and some toasted bread cubes.