Cook 3/4 pound spaghetti, reserving 1 1/2 cups pasta water, then drain. Meanwhile, in a skillet, cook 2 sliced onions in 2 tablespoons olive oil until slightly golden. Add 1 pound halved cherry tomatoes and cook until just soft. Add pasta water and 2 ounces grated pecorino cheese and heat until thick and creamy. Add pasta, 1/4 cup rinsed capers, and 2 cans drained oil-packed tuna. Cook until sauce coats pasta. Season with lemon zest and juice. Top with toasted panko.