Prepare the au jus by heating the olive oil in a small saucepan, then add shallot once it's hot. Sauté for 1-2 minutes, then add garlic. Cook for 60 seconds, stirring frequently.
Add broth, bouillon crystals, thyme, sea salt, and freshly cracked black pepper to taste.
Simmer for at least 15 minutes for flavors to combine. Taste and re-season if needed.
Meanwhile, prepare the slightly caramelized onions by heating the butter in a large skillet over medium heat. Add the onions and sauté for 20 minutes, stirring often, until golden brown and caramelized. Season with salt, remove from heat, and set aside. Side Note: Add a little water to the large skillet if it and the onions become dry.
Toast the buns under the broiler in your oven.
Prepare the French dip sandwiches by placing the roast beef into the au jus, then remove after 20 seconds.
Let the juices drip back into the au jus pan, then place on the hoagie.
Add a spoonful of onions and top with the other bun.