Place eggplant slices on a wire rack; sprinkle with salt. Let sit until some liquid starts beading on slices, about 20 minutes. Rinse salt off eggplant slices and pat dry. Brush olive oil over both sides of each slice. Arrange eggplant slices on a baking sheet.
Roast in the preheated oven for 10 to 15 minutes. Flip eggplant slices and continue roasting until slices are tender and lightly browned, 15 to 20 more minutes. Remove eggplant from oven and reduce temperature to 350 degrees F (175 degrees C).
Mix ricotta cheese, Italian cheese blend, egg, garlic, oregano, thyme, and black pepper together in a bowl.
Line a 9-inch square pan with aluminum foil. Coat the bottom of the pan with about 1/2 cup tomato sauce; top with 1/2 of the eggplant slices. Layer eggplant slices with 1/2 of the spinach, 1/2 of the ricotta mixture, and 1/2 of the remaining tomato sauce. Continue layering with remaining ingredients, ending with sauce. Top with mozzarella slices. Cover pan with aluminum foil.
Bake in the preheated oven for 30 minutes. Remove aluminum foil and continue baking until cheese is brown and bubbling, 5 to 10 more minutes.