Meanwhile, in a large dutch oven, melt the butter and add the leek, onion, fennel and season with salt and pepper. Cook over medium heat until the veggies are soft., about 8 minutes. Whisk in the flour until all has combined, and cook or 3-4 minutes. Gently whisk in the vegetable broth and add the thyme, oregano, mushrooms, parsley, carrots, peas and cooked potatoes. Stir to combine then season again with salt and pepper. Slowly drizzle in the heavy cream or milk and stir again to combine. Let simmer, covered, for about 20-25 minutes.