Note: When using store-bought stock, please reduce to a third of the original volume. For example, for 6 cups of stock use 18 cups of store-bought beef stock and reduce.
Preheat oven to 425º F and roast veal bones for 30 minutes or until golden brown, turning halfway.
Roast the mirepoix or sear it in batches until it is golden brown.
Caramelize the garlic in grapeseed oil and butter until it is golden brown.
Place the mirepoix, garlic (with the grapeseed oil and butter), and the tomato paste in a stock pot large enough to fit all the ingredients and cook until the tomato smell is out of the paste and it is a rust color.
Add the bay leaves, thyme, bones, and water to the stock pot.
Bring up to a boil, turn to a low simmer and allow to cook overnight or at least 8 hours.
Strain through a chinois and ensure there is 4 QT of stock. If it is low add water.
Tie the garlic clove, bay leaf, and thyme in a sachet of cheesecloth with twine. Set aside.
Remove the center germ of the onions and slice the onions into ½-inch thick slices.
In a cold heavy-bottom large saucepan, add enough grapeseed oil to coat the bottom.
Add all the sliced onions to the cold saucepan. Be sure to separate all the pieces. Cook on high heat for about 10 minutes, stirring occasionally so the onion does not burn. Reduce heat to medium to caramelize gradually.
When the onion has caramelized to a golden brown color, add 2 oz of butter to the saucepan and salt to taste. Deglaze with dry sherry, add the beef stock and simmer 20-30 minutes with the cheesecloth sachet of aromatics.
Add Worcestershire sauce.
Pour the soup into oven-safe bowls, top with croutons, and 4-5 slices of cheese.
Place the soup bowls on a baking tray in the oven and set to broil so that the cheese melts over the edges of the bowl and caramelizes on the top.