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.