Heat the olive oil in a sauté pan over medium heat. Add the onions and cook until soft and translucent.
Add the balsamic vinegar to deglaze the pan, then stir in the brown sugar. Reduce the heat to medium low and cook for 20 to 25 minutes, stirring frequently, until the onions are deeply caramelized and reduced.
Herbed Butter
Combine the thyme, rosemary, roasted garlic and olive oil in a blender or food processor and pulse to form a purée.
Combine the softened butter, herb purée, parsley, scallions and sea salt. Whip until light and fluffy. Refrigerate until ready to use.
Herbed Beef Broth
Preheat oven to 350°F. Toss the beef bones with the tomato paste and place on a baking sheet. Roast for 15 to 20 minutes, or until browned.
Transfer the bones to a large pot and add the water. Bring to a gentle boil, then immediately reduce to a simmer. Skim off any foam that rises to the surface.
Simmer gently for 6 to 8 hours, uncovered or partially covered.
Remove the bones and add the onion, carrot, celery, garlic, thyme, rosemary, bay leaf and black peppercorns. Simmer for another 1 to 2 hours.
Remove the aromatics and strain the broth through a fine mesh sieve. Stir in the lemon juice and sea salt, adjusting the seasoning to taste.
Cool and refrigerate the broth until fully chilled. Remove the solidified fat from the surface before reheating.
Sandwich
Cut the bread into 4 sandwich portions, then slice each portion in half lengthwise. Spread herbed butter over both cut sides.
Spread caramelized onions over one side of each sandwich and top with Mountain Jack cheese. Divide the roast beef among the other halves.
Place open faced under a broiler or in a toaster oven until the cheese is melted and bubbly and the edges of the roast beef begin to crisp.
Close each sandwich.
To Serve
Serve the sandwiches warm with individual cups of hot herbed beef broth for dipping.
Notes
Store bought beef broth can be substituted for the homemade broth. To add more flavor, simmer it for 5 to 10 minutes with rosemary, a smashed garlic clove and a few black peppercorns, then strain before serving.