Heat a grill or grill-pan to medium.
In a small bowl soak the onions in cold water to cover for 10 minutes. Drain.
Brush the tuna with olive oil and season with salt and pepper. Grill the fish, turning once, about 5 minutes per side. Set aside.
In a large bowl toss the arugula with a couple tablespoons of the vinaigrette, and divide the greens onto 4 dinner plates. In the same bowl combine the green beans, artichokes, potatoes, onion, tomatoes, and fennel. Dress the vegetables with a 1/2 cup of the dressing, season with salt and pepper. Place vegetables on the greens and top the salad with the tuna steaks. Drizzle a little dressing over the tuna and serve. Pass the remaining dressing at the table.
In a small bowl, whisk the mustard, vinegar, salt, and pepper together. Combine the oils in a cup with a spout. Gradually pour the oil into the vinegar mixture while whisking constantly to make a creamy dressing.
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