Remove scales and wash thoroughly. Place fiddleheads in a vegetable steamer over boiling water sprinkle garlic over them.
Cover and steam 20 minutes or until tender, but still crisp. Set aside, and keep warm.
Combine cornstarch and buttermilk in a small saucepan; stir well. Cook over medium heat until thickened and bubbly, stirring constantly. Remove from heat; stir in mustard, lemon juice, tarragon and pepper.
Arrange fiddleheads on a serving platter, spoon sauce over fiddleheads. Granish with lemon slices.