Heat a large pot over medium heat. Add butter and melt until foamy. After trimming any gristle from oxtails, dredge in flour and season with salt and pepper. Place floured oxtails in butter and brown on all sides. Remove from pot.
Add onions to pot and caramelize over low heat, taking care not to burn. When onions are brown, add carrots, celery, turnips and garlic and sweat mirepoix together. Add browned oxtails back to pot. Sprinkle oxtail mixture with rosemary, savory, mustard, tomato paste, salt and pepper. Add beer, stock and water. Bring the mixture to a boil and reduce to a simmer. Skim fat from soup. Continue to simmer for 2 hours on low heat.
Optional: Cooked meat may be stripped from bone and added back to soup or eaten as is from the bone.