To prepare a spinach fettuccine with clam butter sauce and diced tomatoes recipe, first you need to prepare the spinach. First, you'll need 2 cups of thawed spinach, 2 tablespoons of butter, 1 teaspoon of salt, and 1 tablespoon of freshly ground black pepper. If you don't have fresh spinach you can use frozen, but if you want to make your own, it's best to use fresh. You can also buy frozen spinach at your local grocery store. To make the sauce, heat the butter over medium-high heat until it starts to foam. Then, add the garlic and cook until fragrant, about 1 minute. Add the pasta and toss to coat the noodles. Cover the pot and let it cook, covered, for 5 minutes. Remove the pan from the heat and stir in the Parmesan cheese and salt and pepper to taste. Serve immediately.