If you can't find parsnips, carrots make a terrific substitute.
Arrange parsnips in a steamer basket over boiling water. Cover and steam 8 minutes. Add broccoli; cover and steam 5 minutes. Place parsnips and broccoli in a large bowl.
Melt butter in a small saucepan over low heat. Cook 5 minutes or until lightly browned. Remove from heat. Stir in horseradish, lemon juice, and salt. Pour sauce over vegetables, tossing to coat. Sprinkle with walnuts. Serve immediately.