In a covered large saucepan cook sliced potatoes in a small amount of boiling, lightly salted water about 10 minutes or just until tender, adding the leek the last 5 minutes of cooking. Drain potato mixture.
Coat a 2-quart rectangular baking dish with cooking spray. Place potato mixture in bottom of the prepared baking dish. Sprinkle with the ham and Swiss cheese.
In a medium bowl stir the milk into the flour until smooth. Stir in the egg product, thyme, and pepper. Pour the egg mixture over potato mixture.
Bake in a 350 degrees oven for 35 to 40 minutes or until a knife inserted near center comes out clean. Serve immediately.