Cook shallots in butter in a 12-inch heavy skillet over medium heat, stirring frequently, until just tender, about 4 minutes. Stir in asparagus, peas, sea salt, and 1/4 teaspoon pepper, then seal skillet with foil. Cook over medium heat until vegetables are tender but still slightly al dente, about 8 minutes. Stir in basil and sea salt to taste.