Drain clams and reserve clam juice. In a large nonstick skillet, heat olive oil over medium-high heat, add onion, and cook until soft, about 5 minutes. Stir in garlic and cook 30 seconds more. Add tomatoes, stir, and cook for 1 minute. Stir in reserved clam juice, broth, and lemon juice; bring to a boil. In small bowl, combine cornstarch with 1 tablespoon water, and whisk into boiling broth mixture; boil 1 minute. Reduce heat to low, and stir in clams and parsley.

Cook linguini and combine with clam sauce. Slice iceberg lettuce into wedges and serve as a side salad with pasta.