Pat the chuck steak dry with paper towels, then season both sides with salt and black pepper. Heat 1/2 tablespoon of olive oil in a Dutch oven over medium-high heat. Once hot, sear the steak for about 3 minutes per side, until nicely browned.

Add garlic cloves, 1/3 of the sliced onions, rosemary, thyme, beef bouillon, soy sauce, mushroom umami seasoning, beef stock, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Bring to a boil, then cover and simmer on low for 2-3 hours, until the beef is tender.

Taste the broth and adjust seasoning with more salt and pepper if needed. Remove the steak from the pot, shred the meat using two forks, and set aside. For the au jus, ladle the simmered broth through a fine-mesh strainer to remove the aromatics and bits. Set the strained sauce aside for dipping.

Trim off the ends of the French baguette, then cut into 4-inch segments. Slice each one open, butter the cut side, and toast them on a skillet until slightly golden brown. Spread about 1 tablespoon of aioli on each sandwich, then top with shredded beef, caramelized onions, and 1–2 slices of provolone cheese.

Broil the sandwich on high for 1-2 minutes, cheese side up, until melted. Garnish with chopped chives, if desired. Serving warm with the au jus sauce, and enjoy!