This is quite a rich dish, rich and delish. You can substitute any number of greens, just cook them to your preference before baking. Other cheeses would work, too–Gruyère would be especially tasty, as would Edam or Gouda. I think you could use non-dairy liquids in place of the cream; though the dish would probably be somewhat looser, less thick.
4 T butter, or oil, or a combination, divided
3 T prepared mustard, divided
2 c fresh breadcrumbs or wheatgerm
1 c crumbed blue cheese, or other (grated, shredded, diced) cheese
1 1/2 – 2 lbs spinach or other greens, washed and coarsely chopped
3 c thinly sliced leeks, white and light green parts only; or onion or scallion
3/4 c heavy cream, light cream, or whole milk
salt and pepper, to taste
grated nutmeg, to taste, optional
1. Preheat oven to 400°F. Melt 2 T butter or heat 2 T oil in a large skillet over medium heat. Stir in 2 T of the mustard, then add the breadcrumbs. Sauté until breadcrumbs are golden, if using white breadcrumbs, or have turned a darker shade, if using whole grain breadcrumbs, about 5 minutes. Remove from heat, transfer to a bowl, let cool some, then mix in the cheese.
2. In the same skillet, add another T of butter or oil, if needed, and sauté spinach until wilted or to your preference. You may need to do this in batches. Transfer the greens to a colander to drain. Press on the greens to remove excess liquid.
3. Melt remaining butter or heat remaining oil in this skillet. Add leek and sauté about 4 minutes, or about 8 minutes for regular onion. Add the cream and remaining T of mustard. Stir to blend. Add greens. Cook, stirring, until the cream thickens a bit, a couple of minutes–no need to bring to the boil. Season with salt, pepper, and grated nutmeg, if using. Transfer to an appropriate baking dish. Scatter breadcrumb mixture over. Bake until bubbling, 10 – 15 minutes. Remove from oven and let stand a couple of minutes before serving.