1. Heat oven to 350 degree F. Lightly coat 13x9-inch baking dish with vegetable cooking spray; set aside.
2. Discard green portions from leeks. Slice white portions of each lengthwise in half; cut crosswise into slices. Rinse leeks well to remove all sand; drain.
3. Heat water and corn-oil spread in Dutch oven or large nonstick skillet over high heat. Add leeks, sliced onions, and 1/2 teaspoon salt; toss to coat well. Reduce heat to low; cover and simmer, stirring until leeks and onions are tender, 10 minutes.
4. Stir in ham, 3/4 teaspoon salt, the pepper, and thyme. Cook, uncovered, stirring over medium-high heat, until liquid is evaporated and leeks and onions are golden.
5. Meanwhile, thinly slice potatoes.
6. Place flour in large bowl; gradually whisk in milk until smooth. Gradually stir into leek-onion mixture. Bring to boil over high heat, stirring until thickened; boil 1 minute.
7. Spoon 1/2 cup of the ham and leek mixture into bottom of prepared pan. Top with half the sliced potatoes. Spoon half of remaining ham and leek mixture over potatoes. Arrange remaining potatoes and cover with remaining ham and leek mixture.
8. Cover dish with foil and bake 1 hour 15 minutes. Remove foil; sprinkle with parsley and bake, uncovered, 15 minutes more or until potatoes are tender. Makes 8 main-dish servings.