Heat olive oil in heavy pot over low-medium heat. Add onion, garlic, pappers, salt and pepper. Cover, and cook, stirring ocasionally (~15 minutes) until vegetables have released some liquids
Add tomatoes and soy beef. Using a potato masher, break up ground soy beef into small pieces. Cover and cook for about 20 minutes, stirring ocasionally.
Add the beans, mustard, and oregano. Stir well, to combine. continue cooking, uncovered, for about 10 minutes, stirring ocasionally until heated through.
Taste for seasonings.
Spoon filling into buns and serve immediately.