In a large serving bowl, mash together lightly half the roe, two of the tomatoes, and the minced garlic. Mix in 1/3 cup olive oil and salt and hot pepper to taste.
Cook the spaghetti in boiling salted water until al dente. Drain; toss quickly with the roe mixture. Drizzle with the remaining oil if the pasta seems dry. Top with the remaining roe and the remaining tomato. Sprinkle with the parsley and garnish with the lemon halves. Serve immediately.