How to Make It
In a medium saucepan, boil the wine until reduced to 1/3 cup. Add the demiglace and cook until reduced to 1/2 cup, about 10 minutes longer; remove from the heat. Add the olives and season with salt and pepper. Cover and keep warm.
In a large cast-iron skillet, heat 1 tablespoon of the olive oil until almost smoking. Add the onion and cook over moderate heat, stirring frequently, until softened and browned, about 10 minutes. Transfer the onion to a bowl. Add another tablespoon of the olive oil to the skillet along with the bell peppers and cook over moderate heat, stirring frequently, until softened and browned, about 10 minutes. Add the peppers to the onion and season with salt and pepper.
Wipe out the skillet. Heat another tablespoon of the olive oil over high heat until shimmering. Season one-third of the steak slices with salt and pepper and add them to the skillet in a single layer without crowding. Cook until browned on the bottom, about 30 seconds. Flip the steak slices and cook for about 20 seconds longer for medium; transfer the meat to a plate and keep warm. Heat the remaining 1 tablespoon of olive oil in the skillet, season and cook the remaining steak in 2 batches.
Arrange the seared steak slices on the bottom halves of the baguettes and drizzle any accumulated juices on the cut sides of the top halves. Top the steak with the onion and peppers, the spinach and the wine and olive sauce. Close the baguettes, cut each in thirds and serve.
Beef and veal demiglace are available at specialty food shops.