Preparation

• In a large bowl, whisk 2 Tbs. of the olive oil with the vinegar, mustard, 1/2 tsp. salt, and 1/2 tsp. pepper. Set aside.
• Bring a large pot of well-salted water to a boil. Cook the spaghetti according to package directions for al dente, adding the green beans to the water when 2 minutes remain.
• Drain the spaghetti and beans and toss with the vinaigrette. Add the tomatoes, olives, capers, and basil, and toss. Season to taste with more pepper.
• Season the tuna on all sides with salt and pepper. In a large heavy-duty skillet, heat the remaining 1 Tbs. olive oil over high heat until shimmering hot. Sear the tuna on all sides, about 30seconds per side. Transfer to a cutting board, tent with foil, and let rest for 5minutes. Cut crosswise into 1/4-inch-thick slices.
• Divide the pasta and vegetables among four wide, shallow bowls, top with the tuna, and garnish with the basil leaves.