Preheat oven to 375°F. Coat a large nonstick skillet with oil spray.
Heat skillet over medium heat. Add onion. Cook for 6 to 8 minutes or until tender, stirring occasionally. Add garlic, thyme, ½ teaspoon of the salt, and the pepper.
Cook and stir for 30 seconds. Transfer onion mixture to a blender.
Add the undrained tomatoes and tomato paste. Cover and blend until smooth.
In a large pot, cook the lasagna noodles 1 minute less than the time given on the package (it should be about 9 minutes).
While the lasagna is cooking, add the eggplant slices, in three batches, to the boiling water and cook for 3 minutes per batch (the eggplant will float on top of the lasagna noodles).
Remove the eggplant slices from the water and drain in a large colander. Drain the lasagna when it is done cooking.
In a medium bowl beat egg whites lightly with a fork. Stir in the remaining ¼ teaspoon salt, the ricotta cheese, Parmesan cheese and half of the crumbled feta cheese.
Spread about ½ cup of the tomato sauce in the bottom of a 3-quart rectangular glass baking dish. Top with 3 of the noodles. Top with one-third of the ricotta mixture and spread to an even layer. Top with one-third of the eggplant slices.
Pour one-third of the remaining tomato sauce over the eggplant. Repeat layers twice, starting with lasagna noodles and ending with the tomato sauce.
Bake, uncovered, for 55 to 65 minutes or until lasagna is heated through (an
instant-read thermometer should read 160°F in the center of the lasagna). Let stand for 15 to 20 minutes before serving.
Cut lasagna into 8 servings and place one serving on each of 8 dinner plates. Sprinkle with remaining crumbled feta cheese before serving.