Place tempeh in a medium-sized saucepan with water. Cover and bring to a simmer, and cook for 10 minutes. Drain tempeh and pat to remove excess water. Set aside.
Place oil in a large skillet or wok over medium heat. Add tempeh and cook, turning, until brown on both sides, about 5 minutes. Remove with a slotted spoon and set aside.
Add the onion, garlic and ginger to the skillet. Cover and cook, stirring a few times until softened, about 5 minutes.
Stir in the tomatoes, molasses, mustard, tamari, vinegar and cayenne and bring to a boil. Reduce heat to low and simmer, stirring occasionally, to thicken the sauce slightly and develop the flavors, about 13-15 minutes.
Return tempeh to the sauce, and cook for 10 minutes longer. Serve warm.

1 pound (16-ounce) tempeh, cut into 1/2-inch-wide strips
2 tablespoons olive oil
1-2 garlic cloves, minced
1 small yellow onion, minced
1/8 teaspoon cayenne
1 1/2 teaspoons fresh ginger, peeled and minced
1 (14-ounce) can crushed tomatoes
1 tablespoon Dijon mustard
3 tablespoons tamari or soy sauce
1/4 cup unsulphured molasses
1 tablespoon rice vinegar