Grilled tuna salad nicoise

1/4 cup fresh squeezed lemon juice
1/2 teaspoons salt
1 teaspoon Dijon mustard
1/2 cup extra-virgin olive oil
Freshly ground black pepper, to taste
2 tablespoon capers, drained
1 tablespoon chopped fresh basil leaves
2 1-inch thick tuna steaks, 6 to 8 ounces each
2 medium new potatoes, scrubbed well
1/2 pound asparagus spears, trimmed
Olive oil, for potatoes and asparagus
8 ounces mixed salad greens, washed and dried
10 cherry tomatoes, washed
10 Nicoise or Greek olives, drained

Whisk together lemon juice, salt, Dijon mustard, olive oil and black pepper in a small bowl. Place tuna steaks in a zip-top plastic bag and add 1/2 cup of the lemon juice mixture. (Reserve the remaining vinaigrette to dress the salad greens.) Squeeze bag to distribute marinade evenly around steaks. Let sit at room temperature 20 minutes. Meanwhile, place potatoes in a saucepan and cover with water. Add a dash of salt and bring to a boil. Reduce heat and simmer until the potatoes are tender, about 20 minutes. Drain potatoes and set aside to cool. Slice potatoes into 1/2-inch thick slices. Brush both sides of the potatoes with olive oil and season with salt and freshly ground black pepper. Toss the asparagus spears with olive oil and season with salt and freshly ground black pepper. Preheat range-top grill on high setting for 5 minutes. Remove steaks from marinade and grill 3 to 4 minutes per side. Place the sliced potatoes and asparagus spears on the grill and grill for 3 to 4 minutes per side. Line serving plates with the mixed greens. Add the capers and chopped basil to the reserved vinaigrette. Top the greens with the grilled asparagus, new potatoes and tuna steaks. Distribute the cherry tomatoes and olives evenly around plate and top with the vinaigrette.