1. Preheat oven to 375° F.
2. Clean leeks. Trim the root end of each leek and peel off a couple outer layers. Slice halfway through along the length of each leek. Rinse under running water and use your hands to separate the layers and rinse away any dirt. Slice the white and light green parts into ½-inch rounds, peeling off dark green parts as you go to reveal more light green layers.
3. Melt butter in large skillet over medium heat. Add leeks and sauté for about 10 minutes, stirring occasionally until leeks are soft and starting to brown a little.
4. Make the topping while the leeks are cooking. Mix the flour, salt, pepper, and poultry seasoning in a large bowl. Cut the butter into the flour with a fork or your fingers (mashing the butter and flour together until crumbly), then add the oats, pecans, and cheese. Use a fork to mix all ingredients together.
5. Once the leeks are soft, add salt and garlic and cook one minute, stirring frequently. Add thyme and heavy cream and scrape any browned bits from the bottom of the pan.
6. If your skillet is oven-proof, use it or add the leek mixture to a 9 x 9-inch baking dish/gratin dish and spread the topping evenly over the leeks.
7. Bake for 35 minutes, or until bubbly and the topping is lightly browned. Serve hot.