Preheat an oven to 450 degrees F (230 degrees C). Line a baking sheet with foil.

Spread out onions, carrots, and celery on the prepared baking sheet. Set rib bones on top.

Roast until bones are deep brown and onions are black, about 1 hour. Use a spatula or tongues to transfer bones and vegetables into a saucepan. Drain off and reserve any rendered fat from the baking sheet for the gravy.

Add broth and water to the saucepan, and bring to a boil on high heat. Reduce heat to low and simmer until meat falls off bone and is flavorless, about 4 hours. Skim any fat from the surface and reserve. Strain broth and let cool to room temp.

Melt butter in a pot on medium-high and sauté mushrooms until well browned. Add 2 tablespoons rendered beef fat and stir until melted. Add flour and cook for a few minutes. Whisk in broth, and bring to a simmer. Cook, stirring occasionally, about 5 minutes. Reduce heat to medium and simmer until gravy has thickened to desired thickness, 5 to 10 minutes more

Adjust salt to taste and season with freshly ground black pepper, cayenne, and balsamic vinegar. Taste again, adjust, and serve.