1. In a 5- to 6-quart pan over high heat, bring about 4 quarts water to a boil. Add penne and cook, stirring occasionally, until tender to bite, 10 to 12 minutes. Drain.
2. Meanwhile, in a 10- to 12-inch frying pan (with sides at least 2 in. tall) over medium heat, stir onion and garlic in olive oil until limp, 3 to 5 minutes.
3. Add lemon peel, lemon juice, butter, and scallops to pan with onions. Bring to a simmer and cook until scallops are opaque but still moist-looking in the center (cut to test), about 6 minutes.
4. Add pasta and basil to scallop mixture and stir until heated through. Add salt and pepper to taste. Pour into a wide, shallow bowl to serve.