Trim dark-green parts from leeks; slice leeks crosswise, and soak in cold water. Swish leeks to help loosen dirt. Repeat with fresh water until you no longer see any grit at bottom of bowl. Lift leek slices out of water, leaving grit behind; drain them thoroughly on paper towels.
Preheat oven to 450 degrees. Place mushroom caps, gill sides up, on a rimmed baking sheet; drizzle with oil and vinegar, and season with salt and pepper. Bake until caps are just tender, about 15 minutes.
Top with leeks, spinach, and cheese; season again with salt and pepper. Bake until spinach wilts and cheese starts to brown, 10 to 12 minutes.
WOW.
i also recommend cooked spinach - my fresh spinach wilted and "dried", making it not as pretty as the image above. it was also more difficult to add the cheese on top with fresh spinach!
I actually think this is a lot easier with cooked spinach. When you stack the leeks, goat cheese and fresh spinach, it gets a little unruly.