In medium pot over medium-high heat, add the bacon and cook for 3 minutes, stirring occasionally. Add the diced tomatoes and cook for 2 minutes. Add the shallot and garlic and cook for another 2 minutes. Stir in the tomato paste. Pour in the broth and bring to a boil over high heat. Once it is boiling, remove from heat and let it cool for 5 minutes.
In a blender, process the broth mixture until smooth. You may want to do it in 2 rounds. Pour the creamy broth back to the same pot. Add onion. Bring back to a boil over medium-high heat. Season with salt and pepper.
Meanwhile, bring a large pot of water to a boil. Follow the udon package direction to cook the udon. Drain and divide the udon into 2 large serving bowls.
In a medium bowl, add the sliced beef. Season with salt. Add the sesame oil. Mix together. Place the beef into the tomato broth. Stir in the beef slices and cook until just cooked through. Remove the beef and onion. Place them on top of the udon. Scoop broth over. Top with green onions and tomato wedges. Serve immediately.