Let roast sit at room temperature about an hour
Preheat Oven to 500 degrees
Make compound butter by adding 2 softened sticks of butter, 7-8 cloves of garlic, and equal parts of chopped rosemary, thyme, and oregano into a food processor
Salt and pepper the roast on all sides, then coat it in the compound butter
In a roasting pan, place 2 halved yellow onions to hold the roast above the broth, then add celery, carrots, 2 heads of garlic with tops cut off, green onions, and any leftover herbs, and 4 cups of beef broth
Roast for 5 minutes per pound, then turn off the heat and leave in the oven for 2 hours without opening
In a large skillet, cook 2 red onions and 2 yellow onions in 1 stick of butter and olive oil for about 45 minutes, adding Worcestershire sauce and a splash of red wine
Remove the roast from the oven and let it rest on a cutting board
Place the roasting pan on the stovetop, add 2 more cups of beef broth, and simmer to make the au jus
Puree the roasted garlic from the pan in a bowl, then mix in 1 cup of mayo, 1 tablespoon of prepared horseradish, and a splash of Worcestershire sauce
Toast the bread, assemble the French dip with the roast beef, sautéed onions, au jus, and provolone cheese, then broil for a couple minutes
