Heat the oven to 425°F. Put a small rimmed baking sheet in the oven. Rinse the squash, halve them lengthwise, and scoop out the pulp and seeds. Rub the halves all over with 1 tablespoon oil and sprinkle with salt and pepper. Put the squash on the baking sheet, cut side down, and roast until they’re slightly browned on the cut side, 10 to 15 minutes. Rinse the quinoa in a strainer and put it into a small saucepan with 1 3/4 cups water and a pinch of salt. Bring to a boil and then reduce the heat to medium-low. Cover and bubble gently until the quinoa has absorbed all of the water, 15 minutes or so. Remove the pan from the heat and let it rest, covered, for 5 minutes.
Rinse, trim, and slice the mushrooms. Put 1 tablespoon oil in a medium skillet over medium-high heat. When it’s hot, add the mushrooms, sprinkle with salt and pepper, and cook, stirring occasionally until they’re crisp in places, 5 to 10 minutes. Rinse the rosemary, strip the leaves from the stem, and chop them. Chop the walnuts. Rinse and dry the greens. When the mushrooms are ready, put them on top of the quinoa and cover the pot again. Return the skillet to medium-high heat. Add the bread crumbs, rosemary, and a little salt and pepper and cook, stirring frequently and adjusting the heat so they don’t burn, until they’re crisp and sizzling, 3 to 5 minutes.
When the squash begins to brown on the cut side, turn
them and continue to roast until barely tender, 5 to 10
minutes more. Add the walnuts to the mushrooms and
quinoa; fluff with a fork, mixing gently to combine.
Rinse and halve the lemon; squeeze the juice from one half into a large bowl (save the other half for another use) and add the remaining tablespoon oil. Sprinkle with salt and pepper and stir to combine. Add the greens and toss until coated with dressing.
Taste the quinoa and adjust the seasoning. To serve, put the squash on plates, cut side up, spoon some of the quinoa filling on top, and sprinkle with bread crumbs. Pass the salad and extra filling and bread crumbs at the table.