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1
Make the jus In a saucepan, melt the butter.
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2
Add the onion and cook over moderate heat, stirring, until starting to brown, 10 minutes.
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3
Add the sugar, garlic, thyme and bay leaves and cook over moderately low heat, stirring, until the onion is deep golden, about 20 minutes.
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4
Add the port and simmer until nearly absorbed, 2 minutes.
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5
Add the broth, vinegar and Worcestershire and simmer over moderate heat until reduced to 2 cups, about 30 minutes.
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6
Strain the jus into a saucepan and season with salt and pepper.
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7
Roast the beef Preheat the oven to 325.
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8
Heat the oil in an ovenproof skillet.
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9
Season the meat with salt and pepper and sear until well browned all over.
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10
Add the butter, garlic and thyme and baste the meat for 1 minute.
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11
Transfer to the oven and roast for 30 minutes, basting every 5 minutes, until an instant-read thermometer inserted in the center of the meat registers 128; transfer to a rack set over a baking sheet and let rest for 20 minutes.
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12
Add the broth to the skillet and scrape up any browned bits.
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13
Boil until reduced by half; strain into the jus and skim off any fat.
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14
Make the aioli In a food processor, blend the egg yolks, vinegar and a large pinch each of salt and pepper for 90 seconds.
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15
With the machine on, add the oil in a slow, steady stream.
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16
Transfer to a bowl, stir in the horseradish and season with salt and pepper.
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17
Make the sandwiches In a skillet, cook the shallot and garlic in 1 tablespoon of the butter, until softened.
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18
Add the spinach and season with salt.
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19
Cook, tossing, until just wilted; transfer to paper towels to drain.
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20
Make the sandwiches Preheat the broiler.
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21
Thinly slice the beef.
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22
Arrange the baguettes on 2 baking sheets and spread with the aioli.
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23
Spoon the spinach over the bottom halves and top with the beef and cheese.
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24
Broil until the cheese melts.
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25
Make the sandwiches Turn the oven to 425.
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26
Reheat the jus.
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27
Close the sandwiches, brush with the remaining melted butter and weight down with a baking sheet and heavy skillet.
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28
Bake for 10 minutes; remove the weighted baking sheet and bake for 5 minutes.
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29
Serve with the jus.
