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1
Heat a grill pan or outdoor grill to high; preheat broiler.
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2
In a bowl, mix together the garlic, grill seasoning, hot sauce, Worcestershire, and vinegar.
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3
Whisk in 3 tablespoons olive oil.
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4
Put the flank steak in a shallow dish; pour the marinade over the steak.
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5
Toss to coat thoroughly and marinate for 5-10 minutes.
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6
While the steak is marinating, heat a large skillet over med-high heat; add in the remaining olive oil.
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7
Add in the onions, thyme, salt, and pepper; cook, stirring often, for 7-8 minutes or until the onions are deep golden brown.
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8
Add in the chicken stock; continue to cook for 2-3 more minutes or until the chicken stock has almost completely cooked away but the onions are still a little wet with the stock; remove from heat and set aside.
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9
Remove the flank steak from the marinade and pat dry.
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10
Grill the flank steak for 6-7 minutes on each side; remove steak from the grill to a cutting board; cover loosely with foil and let rest 5 minutes.
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11
Thinly slice the steak on an angle, against the grain.
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12
Split the croissants in half and place on a cookie sheet, cut sides up.
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13
Pile some of the onions on the bottoms of each croissant.
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14
Arrange a pile of the sliced steak on top of the onions and then follow that with some more onions.
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15
Place slices of cheese on top and transfer both the croissant bottoms and tops to the broiler.
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16
Melt the cheese and toast the cut sides of the tops 1-2 minutes.
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17
Place the croissant tops on the melted cheese; serve immediately.
