Steam parsnips over medium heat until tender, about 10 minutes. Transfer to paper towels to drain.
Melt 1 tbsp butter in a small skillet over low heat. Add mustard seeds, cover and cook until they begin to pop, about 2 minutes. Remove from heat. Let stand until popping stops, about 8 minutes.
Melt remaining 3 tbsp butter in large nonstick skillet over medium heat. Stir in maple syrup and Dijon mustard. Add parsnips; toss to coat. Sauté until parsnips are glazed, about 8 minutes. Add mustard seeds and toss to coat. Season to taste with salt and pepper. Transfer to platter and serve.