I first read about this tian in a Tom Douglas cookbook (Tom's Big Dinners,) and have since made some version of it, seasonally dependent, nearly once a week. This can easily be made with tomatoes, eggplant, zucchini. Or sweet potato and fennel. Later in the season, I turn to this variation on the theme. —MrsWheelbarrow
Butter a heavy baking dish. I use a Le Creuset 9" oval baker. Double the recipe for a larger Tian, and use a rectangular baker. Preheat the oven to 400°.
Heat 2 T butter in a large skillet until bubbling, stir in the breadcrumbs and then stir together the rest of the ingredients for the crumb topping and set aside.
Separate the squash neck from the bulb. (I use the bulb for soups, first roasting it to make it easier to free the squash from the peel.)
Peel the neck and then slice into thin rounds using a mandoline or a very sharp knife.
Trim and thinly slice the leeks Into rounds. Slice the mushrooms. Mince the shallot.
Blanch the spinach, squeeze dry and chop. (or substitute thawed frozen chopped spinach.)
In a heavy skillet, heat 1 Tbls of butter, add the shallot when it is foaming, then the mushrooms. Cook, undisturbed, until softened and beginning to brown. Remove from the pan and set aside.
Wipe out the skillet and heat 1-2 Tbls. of butter. Add the leeks and cook them slowly until just beginning to brown. Place the leeks on the bottom of the well buttered baking dish.
Add the spinach, sprinkle with salt and pepper and drizzle with a little olive oil. Add the mushrooms, salt and pepper and drizzle again with a little olive oil.
Next, add the squash. Make it decorative. Drizzle with olive oil and sprinkle with salt and pepper.
Bake in the oven for 45 minutes. Remove from the oven and sprinkle on the crumb topping. It will seem like a lot, but it isn't. Reduce the oven heat to 350° and bake for an additional 20 minutes. Serve.