Place the cockles in a large bowl of cold, well-salted water. Soak for 30 minutes, then repeat.

In a small bowl, combine the clam juice and cornstarch.

In a large skillet, add the olive oil and butter over medium heat. When the butter has melted, add the onion and cook for 3 to 4 minutes, until starting to brown. Add the garlic and pepper flakes and cook for another 30 seconds. Add the wine, turn the heat to medium high, and cook for about 5 minutes. Lower the heat to medium, then add the lemon juice and clam juice slurry, and whisk for about 1 minute. Add the tomatoes, parsley, 1/4 tsp. black pepper, and the clams, and cover with a lid. Steam the clams in the sauce until they open, 6 to 10 minutes.

Divide the pasta among four to six serving bowls or place in a large serving bowl, top with the clam sauce and a sprinkle of parsley, and serve.