1. Soak cannellini beans overnight in water.
2. Simmer beans until cooked but tender (approx. 45 minutes).
3. Drain liquid from can of tuna. Break tuna into bite-size chunks.
4. Cut cherry tomatoes in half. Remove stalks from baby spinach and wash twice to remove grit. Dry very well.
5. Cut cucumbers in half lengthways and cut each half in half lengthways again. Cut these strips into 1cm sections.
6. Combine ingredients in a mixing bowl and gently toss.
7. In a small jar combine the mustard, oil and vinegar and shake well. Pour over salad and mix gently.