Fill a medium pot with water. Add potatoes and bring to a boil over high heat. Reduce to a simmer, salt the water, and boil until you can easily sink a knife through the center of the largest potato, about twenty minutes. Drain the potatoes and set aside to cool. Once the potatoes are cool enough to handle, peel them if you like, and cut into quarters.

While the potatoes are cooking, fill a medium pot with water and bring to a boil. Add the eggs and boil for seven to nine minutes. Drain the eggs and rinse with cold water. Once they are cool enough to handle, peel and cut into halves or quarters. Set aside.

Fill a medium pot fitted with a steamer basket with about two inches of water and bring to a boil over medium-high heat. Add the green beans to the basket, cover, and steam until the beans are bright green and tender, five to seven minutes. Carefully remove the beans from the steamer basket and set aside. Add more water if it has evaporated, bring back to a boil, then place the corn in the steamer basket, cover, and steam until the kernels are brightly colored and tender, seven to 10 minutes. Carefully remove from the steamer basket and set aside.

To make the dressing, whisk together the mustard, vinegar, and lemon juice until thoroughly combined. Add salt and pepper to taste. Slowly pour in the olive oil, whisking to emulsify the dressing.

Prepare a large serving platter. Place the open cans of sardines on opposite sides of the platter, then arrange the potatoes, eggs, beans, corn, lettuce, radishes, olives, onion, and tuna on the platter in appealing clusters. Drizzle generously with dressing and serve the salad with the remaining dressing on the side.