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1
In a medium saucepan, boil the wine until reduced to 1/3 cup.
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2
Add the demiglace and cook until reduced to 1/2 cup, about 10 minutes longer; remove from the heat.
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3
Add the olives and season with salt and pepper.
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4
Cover and keep warm.
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5
In a large cast-iron skillet, heat 1 tablespoon of the olive oil until almost smoking.
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6
Add the onion and cook over moderate heat, stirring frequently, until softened and browned, about 10 minutes.
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7
Transfer the onion to a bowl.
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8
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.
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9
Add the peppers to the onion and season with salt and pepper.
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10
Wipe out the skillet.
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11
Heat another tablespoon of the olive oil over high heat until shimmering.
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12
Season one-third of the steak slices with salt and pepper and add them to the skillet in a single layer without crowding.
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13
Cook until browned on the bottom, about 30 seconds.
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14
Flip the steak slices and cook for about 20 seconds longer for medium; transfer the meat to a plate and keep warm.
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15
Heat the remaining 1 tablespoon of olive oil in the skillet, season and cook the remaining steak in 2 batches.
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16
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.
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17
Top the steak with the onion and peppers, the spinach and the wine and olive sauce.
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18
Close the baguettes, cut each in thirds and serve.
