Tuna Nicoise Salad with Parmesan

Name: Tuna Nicoise Salad with Parmesan
Ingredients: 2 tablespoons fresh lemon juice; 1 teaspoon Dijon mustard; 1/2 teaspoon dried oregano; Salt and ground black pepper; 3 tablespoons olive oil; 6 cups mesclun greens; 2 (8-ounce) tuna steaks, grilled; 1/2 cup pitted Nicoise olives, halved lengthwise; 1 cup whole or sliced grape or cherry tomatoes; 1 pound reserved Yukon gold potatoes, parboiled and cut into chunks; 1 cup green beans, cooked; 2 tablespoons drained capers; 1/4 cup chopped fresh parsley leaves; 1/4 cup shredded or grated Parmesan