Preheat the oven 400ºF (200ºC).
Drizzle some olive oil on a rimmed baking sheet, just enough to coat the bottom of the pan. Arrange the squash slices in a single layer. Season with the salt & pepper. Bake for 30 minutes, turning halfway through, until lightly browned on both sides.
Meanwhile, add a swirl of oil to a skillet. Heat over medium flame until shimmering. Add the celery, shallots, shiitake mushrooms, and kale. Season with the salt & pepper. Cook until the vegetables are wilted, and slightly softened. Remove from the heat.
Stir in the quinoa—the heat from the vegetables will warm it through.
Arrange the squash slices on a serving platter. Spoon the quinoa mixture over the squash. Scatter the pistachios on top & serve.
Make Ahead: Both the squash and the filling can be prepared in advance. You can assemble the salad, and serve it cold, if you like. To heat, add the squash to a nonstick skillet, and warm over medium-low heat. Transfer to a serving platter. Add the filling to the same skillet, and heat until warm throughout, then spoon over the squash. Sprinkle the pistachios on top.