Step 1: Saute onion
Stir until onion softens.
Step 2: Add beans & seasonings
Add a nice big handful of coarsely-chopped roasemary, and a handful of chopped garlic. Stir and cook over medium-high heat until beans are warmed through.
Step 3: Add tomatoes and seasonings
Add any other seasonings you like: I recommend a bit of dry oregano and thyme, and possibly a dash of paprika or chili powder. You can add a boullion cube or some anchovy paste for a bit more salt and depth of flavor. Mix everything together and continue to heat.
Step 4: Let it stew
Note that the color changes as the beans and tomatoes both cook down a little bit. Call it done when everything is hot, the tomatoes have softened up, and the liquid has mostly disappeared. You want this to have a stew consistency.
Step 5: Serve
Serve with chopped parsley or basil on top for extra style points. This also goes well on top of pasta, in which case you can leave a bit more juice/sauce on the beans.