Finely dice the red onions, cut the spring onions into thin rings, cut the hard-boiled eggs into large cubes, slice the bell pepper, cut the cucumber into sticks and mix everything with the drained tuna.
For the vinaigrette, mix mustard and honey with olive oil and balsamic vinegar until you get a homogeneous mixture. Then season the salad with salt and pepper and finally add the baby spinach.