Heat a grill pan or outdoor grill to high; preheat broiler.
In a bowl, mix together the garlic, grill seasoning, hot sauce, Worcestershire, and vinegar.
Whisk in 3 tablespoons olive oil.
Put the flank steak in a shallow dish; pour the marinade over the steak.
Toss to coat thoroughly and marinate for 5-10 minutes.
While the steak is marinating, heat a large skillet over med-high heat; add in the remaining olive oil.
Add in the onions, thyme, salt, and pepper; cook, stirring often, for 7-8 minutes or until the onions are deep golden brown.
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.
Remove the flank steak from the marinade and pat dry.
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.
Thinly slice the steak on an angle, against the grain.
Split the croissants in half and place on a cookie sheet, cut sides up.
Pile some of the onions on the bottoms of each croissant.
Arrange a pile of the sliced steak on top of the onions and then follow that with some more onions.
Place slices of cheese on top and transfer both the croissant bottoms and tops to the broiler.
Melt the cheese and toast the cut sides of the tops 1-2 minutes.
Place the croissant tops on the melted cheese; serve immediately.