For the Marinade: In a glass bowl mix together the lemon zest, basil, rosemary, olive oil, and garlic. Add the tuna pieces and mix again. Refrigerate for 1-2 hours.
For the corn vinaigrette: With a large skillet over medium high heat and add 2 tablespoons of olive oil. Add onions and sauté for 3-4 minutes. Add the corn and spring onions and continue sautéing for 3-4 minutes. Add garlic and sauté for 2 additional minutes. Remove from heat and add balsamic vinegar, lemon juice, 6 tablespoons olive oil, basil, thyme, cilantro, salt and pepper. Stir to combine and cool to room temperature.
Prepare the grill. In a stir fry grill pan, cook the tuna stirring constantly until outside of tuna is charred but inside is still rare - about 4 minutes.
On a large serving platter, arrange salad greens, sprinkle with tomatoes, cucumbers and avocado slices. Drizzle the corn vinaigrette over the salad and garnish with cilantro sprigs.