Separate the sliced leeks into rings and soak in a big bowl of cool water for a few minutes to let the grit settle to the bottom of the bowl. Scoop them out carefully and drain.
In a soup pot over medium heat, melt the butter. Add the leeks, potatoes, and celery seed and sauté until the vegetables are slightly softened, about 5 minutes, stirring often to prevent sticking. Add the broth, salt, pepper, and chili powder. Bring to a boil over high heat; reduce to a steady simmer over medium low. Cover and cook until the leeks and potatoes are quite soft, about 15 minutes. Stir in the milk; return to a gentle simmer.
Working in batches, purée briefly in a blender. If the soup is too thick, whisk in more broth until it reaches the consistency you like.
Meanwhile, make the garlic toast: Adjust an oven rack to the top position and heat the broiler to high. Cut the baguette into sharply diagonal slices about 1/2-inch thick (they should be long, narrow slices). Put the bread on a baking sheet, firmly rub both sides with the garlic halves, brush both sides with the olive oil, and season with salt and pepper. Broil until the bread is toasted and crisp, about 2 minutes per side.
Taste the soup for seasonings and serve immediately with the garlic toast.