Place broccoli in a large saucepan; add 1 in. of water. Bring to a boil. Reduce heat; cover and cook for 5-8 minutes or until crisp-tender.
Meanwhile, in a small saucepan, melt butter. Whisk in flour until smooth. Gradually stir in orange juice. Add the orange zest, mandarin oranges and tarragon. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in yogurt.