Whisk together lemon juice and 3 tablespoons extra-virgin olive oil; season to taste with salt and pepper, and set aside. Marinate tomato in 1 tablespoon olive oil, add salt and pepper to taste. Heat remaining tablespoon olive oil in a large sauté pan over high heat, add shrimp and cook for about 2 1/2 minutes, or until golden on both sides. In a large bowl, toss the Mâche with 2/3 of the vinaigrette and arrange on plates. Divide warm shrimp between each salad and garnish with tomatoes. Drizzle remaining vinaigrette over shrimp.