Vegetable Shepherd's Pie

2 lb sweet potatoes, peeled, cut into 2" chunks
1 Tbsp plus 1/2 tsp salt
6 Tbsp butter
1 large yellow onion, thinly sliced
2 parsnips, peeled, sliced 1/4" thick
2 stalks celery, sliced 1/4" thick
2 medium fennel bulbs, cut into a 1/2" dice
2 c Brussels sprouts, halved
2 Tbsp chopped fresh flat-leaf parsley leaves
1/2 tsp black pepper
2 c vegetable broth
3 c fresh spinach or torn Swiss chard leaves
pinch ground nutmeg

Place sweet potatoes and 1 Tbsp salt in a large pot. Add enough cold water to cover; bring to a boil. Reduce heat and simmer until potatoes are tender, about 20 minutes. Meanwhile, in a large saucepan, over medium-low heat, melt 2 Tbsp butter. Add onion and cook 10 minutes. Add parsnips, celery, fennel, Brussels sprouts, parsley, pepper and remaining salt and toss. Add the broth, increase heat, and simmer until vegetables are tender, 15-20 minutes. Add spinach or Swiss chard and stir until wilted. Remove from heat. Using a slotted spoon, transfer vegetables to a casserole or 9" pie plate. Drain the potatoes and return to pot. Add remaining butter and mash until smooth. Spread mashed potatoes over vegetables. Set broiler on high. Broil until lightly browned, 3-5 minutes. Sprinkle with nutmeg.

NOTES
From Real Simple.
Have made a variation of this: potatoes and carrots in place of parsnips, omitting fennel, Brussels sprouts and nutmeg, and adding chickpeas. Chickpeas were a mistake, but otherwise quite good -- using sweet potatoes in place of ordinary potatoes as a topping is an excellent variation on standard shepherd's pie.