1. In a food processor, combine 2 cups spinach leaves, olive oil, garlic, walnuts, and 1⁄4 cup cheese to make the pesto. Process about 1 minute until smooth.
2. Add broth and blend 30 seconds.
3. In a large skillet, over medium-high heat, add pesto mixture.
4. In a small bowl, mix flour and water. Add clams and flour mixture to the skillet. Stir and cook 3 to 4 minutes to thicken and heat through.
5. In a large bowl, toss hot pasta, pesto-clam sauce, remaining 1 cup spinach leaves, and tomato. Top each serving with remaining cheese.
Makes 4 servings.