Saute onion in 3 tablespoons butter in a saucepan over medium-high heat until tender. Add flour, stirring until smooth. Cook, stirring constantly, 1 minute. Gradually add half-and-half; cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Stir in cheese and salt. Cook, stirring constantly, until thoroughly heated. Remove from heat, and stir in lemon juice and dillweed. Cover and keep warm.
Arrange broccoli in a vegetable steamer over boiling water. Cover and steam 5 minutes or until crisp-tender. Transfer broccoli to a serving dish; drizzle with 2 tablespoons melted butter. Spoon sauce evenly over broccoli. Garnish with lemon zest, if desired.