Preheat oven to 325°F.
Sear the beef: Heat avocado oil in a Dutch oven over medium-high. Season chuck roast chunks with salt and pepper. Sear on all sides until browned. Remove and set aside.
Build flavor: Lower the heat to medium. Add sliced onion and shallot; cook 5–7 minutes until softened and golden, add in garlic paste and cook for 30 seconds or so. Deglaze with a splash of beef stock, scraping up browned bits.
Slow roast: Return beef to the pot. Add remaining beef stock, roasted garlic paste, and thyme sprigs. Cover and transfer to a 325°F oven. Cook about 2.5-3 hours, or until the beef is fork-tender.
Shred & strain: Remove the beef, shred into chunks, and strain the cooking liquid. Reserve this liquid as the au jus. (Set aside the onions if you’d like to use them for the sandwiches.)
Toast the bread: Brush baguette halves with avocado oil and spread on garlic paste. Broil until golden and crisp.
Assemble & broil: Layer shredded beef, reserved onions, and the three cheeses on toasted baguette halves. Broil again just until the cheese melts and bubbles.
Serve: Cut into sandwiches and serve hot with a small bowl of warm jus for dipping. Optional but excellent: a side arugula salad tossed with balsalmic vinaigrette.