Tofu or Tempeh Bars

1 pound firm tofu, rinsed, or tempeh
1 medium onion
2 tablespoons oil
2 tablespoons soy sauce
2 tablespoon rice vinegar
4 tablespoons tomato paste
1 1/2 to 2 cups water or broth (more for tempeh)
1/4 teaspoon mustard
pepper
Adapted by Gloria Jimenez from Laurel’s Kitchen