Preheat an indoor or outdoor grill to medium-high. Brush the tuna with 1 Tbsp. of the olive oil and season with salt and pepper. Grill, lightly oiling the surface if prone to sticking, 1 to 2 minutes per side, or until nicely charred on the outside but medium rare in the middle. Remove the tuna from the grill and cool to room temperature. Cut the tuna into small cubes and cover and keep in the fridge until salad is ready. Whisk the remaining oil and the lemon juice, garlic, Dijon mustard, oregano, sugar and salt and pepper in a salad bowl. Add the spinach and toss to coat. Divide the spinach among 4 plates. Decoratively arrange the tuna and remaining ingredients on top of the spinach and serve immediately.
Note: To blanch the green beans, cook in boiling water 1-2 minutes. Drain, cool in cold water, and then drain again. Nicoise olives are sold in the deli department.