In large Dutch oven, heat 1 Tbs olive oil over medium-high heat. Season ribs with salt and pepper. Add half the ribs to the Dutch oven and brown on all sides. Remove ribs to a bowl and brown the other half of the meat. Add them to the bowl and cover with foil.
Add remaining olive oil to Dutch oven and heat. Stir in whole onions, chopped rosemary, salt and pepper. Cook, stirring occasionally until onions are softened, about 5 minutes. Spoon onions and seasonings into a separate bowl and set aside.
Increase heat to high and pour wine into dutch oven. Let boil until reduced by about half, about 10 minutes.
Return ribs and any juices to Dutch oven. Cover and place in oven for 1 hour and 15 minutes.
Add onions to dutch oven, cover and bake for another 30 minutes.
Remove from oven, and transfer onions and ribs to a bowl.
Let cooking liquid stand for a few minutes to allow fat to rise to the top. Skim fat and discard. Bring liquid to a boil and whisk in the mustard and season to taste with salt and pepper.
Add tomatoes and return ribs and onions to Dutch oven. Cover and lower heat to medium and simmer until tomatoes are tender, about 10 minutes more.