For the dressing, whisk together the mustard, vinegar, olive oil, lemon juice, salt and freshly ground black pepper.
For the salad, put the dressing in a frying pan and warm over a low heat. Add the beans, spinach, capers, potatoes and tomatoes and toss them in the warm dressing. Remove from the heat.
Brush the tuna steaks with the olive oil and season with salt and freshly ground black pepper. Heat a griddle pan over a high heat. Cook the tuna steaks on the griddle for about 30 seconds on each side.
To serve, put the tuna steaks on top of the dressed vegetables and top with chopped egg and marjoram.