1. Preheat the oven to 375 degrees. Cook the sweet potatoes in a medium-size pot of boiling salted water until just cooked through, about 3 to 5 minutes. Drain and set aside.
2. In a heavy large saucepan, heat the butter over medium-low heat until melted. Add the onion and cook until soft. Stir in the nutmeg.
3. Slowly stir in the flour until incorporated. Whisk in the chicken stock; simmer until slightly reduced, about 6 minutes. Increase the heat to medium and whisk in 1/2 cup of the Gruyere until melted.
4. In a small bowl, mix the remaining Gruyere with the Parmesan and bread crumbs; set aside.
5. Layer 1 tablespoon of the cheese sauce into each of six 6-ounce ramekins. Layer each with sweet potatoes and more cheese sauce. Top with the bread crumb mixture and bake until the potatoes are creamy and soft, about 20 minutes. Remove from oven and set aside to cool. Garnish with parsley and serve.