Step 1Stir together the yogurt, garlic, dill, tarragon and seasoning. Set aside.
Step 2Bring water to boil in the bottom of a 2-quart saucepan with a steamer insert set over, but not touching, the water. Place the potatoes in the insert, cover and steam over medium-high heat 5 minutes. Add the broccoli crowns; cover, and steam 5 minutes. Add the squash and sausages, cover, and steam another 4 to 5 minutes. Check the vegetables with the tip of a knife -- they should be crunchy yet tender. Steam a few minutes more, covered, if necessary.
Step 3To serve, arrange the vegetables on 2 plates. Spoon the yogurt sauce over the potatoes and season to taste.