How to Make It
Preheat the oven to 300°. In a small saucepan, melt the butter over moderately high heat. Whisk in the flour until blended. Slowly pour in the milk, whisking constantly until smooth, and bring to a simmer. Cook over low heat, whisking often, until no floury taste remains, about 10 minutes. Whisk in the tomato paste until smooth, then season with salt and pepper. Remove the pan from the heat, cover and keep warm.
Meanwhile, in a large skillet, heat 1 tablespoon of the olive oil until shimmering. Add the spinach, season with salt and cook over high heat, stirring, until wilted and any moisture has evaporated, about 3 minutes. Transfer the spinach to a work surface and coarsely chop it. Add 1 tablespoon of the olive oil to the skillet and heat until shimmering. Add the diced zucchini, season with salt and cook over high heat, stirring, until just tender, about 4 minutes. Transfer to a plate to cool. Heat another tablespoon of the olive oil in the skillet until shimmering. Add the diced eggplant, season with salt and cook over moderate heat until browned on the bottom, about 3 minutes. Stir and cook until tender, about 2 minutes longer. Transfer the eggplant to a plate.
In a medium bowl, beat 4 of the eggs with a pinch of salt. In an 8-inch nonstick ovenproof skillet, heat 1/2 tablespoon of the olive oil. Add the chopped spinach and cook over moderate heat until sizzling. Pour in the beaten eggs and stir to combine with the spinach. Cover and cook over moderate heat until the tortilla is partially set, about 5 minutes. Using 2 spatulas, carefully flip the tortilla and cook over moderately high heat until completely set, about 1 minute. Transfer to a large baking sheet. Repeat with 1/2 tablespoon of olive oil and 4 eggs at a time to make 2 more tortillas, one with the zucchini and one with the eggplant. Rewarm the tortillas in the oven.
Reheat the creamy tomato sauce over low heat. Transfer the spinach tortilla to a large cake plate and spread a thin layer of tomato sauce on top. Top with the zucchini tortilla and a thin layer of sauce, then cover with the eggplant tortilla. Pour some of the sauce over the layered tortilla, letting it drip down the sides. Cut the tortilla into wedges and serve with the remaining sauce.