Heat oil in a large skillet over medium heat. Add onion and garlic and saute 3 minutes, until soft. Add roast beef slices and saute 3 to 5 minutes, until browned on all sides, stirring frequently. Add tomatoes, jalapenos, lime juice, and cumin and stir to combine. Bring to a simmer and cook 5 minutes, to heat through and reduce slightly.
Remove from heat and stir in cilantro. Season, to taste, with salt and black pepper. Spoon onto sliced rolls with plenty of sauce and serve.
2006, Robin Miller, All Rights Reserved