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1
Put the balsamic vinegar in a small saucepan over medium heat.
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2
Bring to a simmer and cook until reduced to 1 cup.
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3
Set aside.
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4
Heat 2 tablespoons of the oil over medium-high heat in a heavy-bottomed medium saucepan.
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5
Add the onions and cook until soft, 3 to 4 minutes.
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6
Add the garlic and cook for 1 minute.
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7
Add the ketchup and the water and bring to a boil.
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8
Reduce the heat and simmer for 5 minutes.
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9
Add the reduced vinegar and the remaining ingredients and simmer until thickened, stirring occasionally, for 10 minutes.
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10
Puree with an immersion blender until smooth and season with salt and pepper, to taste.
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11
Pour into a bowl and allow to cool at room temperature.
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12
Sauce will keep for 1 week in the refrigerator stored in a tightly sealed container.
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13
Heat the grill to high.
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14
Brush the flank steak with the remaining 2 tablespoons of oil and season liberally with salt and pepper.
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15
Grill until golden brown on the 1 side, about 4 minutes, turn over and grill until the other side is golden brown, about 4 minutes.
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16
Move to a cooler part of the grill away from direct heat.
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17
Baste with the sauce and turn several more times until cooked to medium-rare doneness, about 15 minutes longer.
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18
Remove from the grill to a cutting board and loosely tent with foil.
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19
Let rest 10 minutes before cutting against the grain into thin slices.
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20
Put the bread on the grill, brush both sides with olive oil and season with salt and pepper.
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21
Grill the bread then remove from the grill.
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22
Top with mounds of grated Monterey jack cheese.
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23
Put the bread slices on a piece of aluminum foil and return to the grill to melt the cheese.
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24
Transfer to a serving platter.
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25
Arrange the flank steak slices on the bread and garnish with sliced scallions before serving.
