Preparation

Make the dipping jus

• In a medium Dutch oven, melt the butter over medium-high heat. When bubbling, add the beef and cook, stirring, until browned all over, about 6 minutes. Add the onion and cook until caramelized, about 8 minutes. Add the garlic, reduce the heat to medium low, and cook for 2 minutes. Pour in the wine, and cook until reduced by half, about 12 minutes. Add the broth, bouillon, and thyme. Simmer on low for 1 hour. Season to taste with salt. Add small amounts of water to thin the jus, if necessary. Place a fine-mesh strainer over a large pot or bowl, and strain the jus, discarding the solids. Set aside. The jus can be made up to 3 days in advance. Store in an airtight container in the refrigerator. Reheat before serving.

Make the flavor paste

• In a small bowl, stir together the chile powder, mustard, Worcestershire, and soy sauce. Set aside. The paste can be made up to 2 days in advance. Store in an airtight container in the refrigerator.

Make the seasoning blend

• In a small bowl, stir together the garlic salt, lemon pepper, black pepper, and cayenne. Set aside. The seasoning blend can be made up to 1 month in advance. Store in an airtight container at room temperature.

Make the honey-garlic glaze

• In a resealable pint-size jar, combine the vinegar and pepper flakes, and let sit for 2 minutes. Add the honey, apple juice, Worcestershire, and garlic to the jar, and then add the butter. Shake well to combine. Set aside. The glaze can be made up to 2 days in advance and refrigerated. If made in advance, warm before using to remelt the butter.

Make the sandwich

• Heat one side of a well-oiled charcoal or gas grill to high and the other to low for a mix of direct and indirect cooking.
• Pat the steak dry with paper towels. Coat the steak all over with the flavor paste, and then coat all over with oil. Sprinkle the steak all over with the seasoning blend.
• Put the steak on the grate heated to high. With the lid open, cook the steak, undisturbed, until well marked and lightly charred, about 3 minutes. Flip the steak, and grill for another 3 minutes. Transfer the steak to the grate heated to low, close the lid, and cook until an instant-read thermometer registers 110°F, about 15 minutes. Meanwhile, position a rack in the center of the oven, and heat the oven to 375°F.
• Shake the glaze, and then brush it all over the steak, reserving some glaze to brush the bread. Continue to brush the steak with the glaze until cooked to your desired doneness, about 5 more minutes for rare, about 6 minutes for medium, and about 8 minutes for well done. Transfer to a cutting board. Let rest for 10 minutes before slicing against the grain into 1/8-inch-thick slices. (Alternatively, cook the steak in the oven. See Tip, below.)
• Cut the baguettes into 5-inch lengths and slice through lengthwise, leaving one side attached so that they open like a book. Lightly toast in the oven for about 5 minutes, then brush the remaining honey-garlic glaze all over the inside of the bread. Divide the steak and caramelized onions among the baguettes. Serve with the warmed dipping jus.