Place the beans in a bowl and cover with cold water. Place in the fridge overnight.
Drain the beans and place them in a medium pot. Add water, covering the beans by at least 5 cm. Bring to a boil, reduce the heat until they are at a nice simmer and cook until tender. Anywhere from 30 to 50 minutes, it depends on the beans.
Preheat your oven to 150 C (300 F). Place the tomatoes on a baking tray. Drizzle with the 15 ml of olive oil and sprinkle with salt. Bake for 35 to 40 minutes, or until they are very tender but not split. Remove and allow to cool.
Whisk the olive oil, lemon juice and salt and pepper in a medium bowl. Set it aside.
When the beans are tender drain them and add to the dressing. Stir and set aside to cool to room temperature.
Add the tomatoes, watercress and onion to the beans. Taste for seasonings.
I used great northern beans but any smallish white bean, like cannellini, would work. I like this salad at room temperature but cold is good too.