Sprinkle eggplant slices generously with salt and let drain in a colander, about 30 minutes. Rinse with water, drain, and pat dry.
Meanwhile, make sauce: Heat 2 tablespoons oil in a medium saucepan over medium heat. Cook onion and garlic, stirring, until translucent, about 6 minutes. Add tomatoes. Bring mixture to a boil; reduce heat and simmer, stirring occasionally, until slightly reduced, about 20 minutes. Remove from heat and stir in oregano.
Preheat oven to 350 degrees and prepare seitan and eggplant: In a shallow dish, mix breadcrumbs and nutritional yeast; season with salt and pepper. Coat both sides of seitan in the mixture, shaking off excess breadcrumbs. Heat 2 tablespoons oil in a large skillet over medium heat. Cook breaded seitan in a single layer until golden brown, about 6 minutes per side. Remove and set aside. Wipe pan clean.
Meanwhile, stir together creamer and flaxseed meal in a shallow dish. Dip eggplant in creamer mixture, drain slightly, and individually coat each piece with breadcrumb mixture, pressing to coat both sides and shaking off excess breadcrumbs. Heat 2 tablespoons oil over medium heat. Cook half the eggplant in a single layer (adding more oil as necessary) until golden brown, about 6 minutes per side. Wipe pan clean and repeat with remaining eggplant, adding more oil.
Spread 1 cup marinara sauce in an 8-by-8-inch baking dish. Layer half the eggplant, followed by the seitan, spinach, and remaining sauce, and top with remaining eggplant. Cover with foil and bake until bubbling, 50 minutes. Remove foil and bake 10 minutes more. Let rest 10 minutes before serving.