Melt the butter in a small skillet or saucepan over medium-low heat. Cook, swirling occasionally, until the milk solids turn light brown, about 5 minutes. Remove from the heat, add the lemon zest and juice, and swirl to combine.
Heat the oil in a 12-inch nonstick skillet over medium heat. Add the squash and 1/2 tsp. salt; toss well to coat.
Cover the pan with the lid ajar by about 1 inch. Turn the heat down to medium low and cook, gently stirring occasionally, until the squash begins to brown, 5 to 6 minutes. As the mixture cooks, you should hear a gentle sizzle.
Turn the heat down to low, add the onions and raisins and cook, stirring occasionally and then gently pushing the vegetables back into a single layer so that most have direct contact with the pan, until the vegetables are tender and browned, 12 to 15 minutes more.
Add the spinach and lemon butter and toss gently until the spinach is wilted, about 1 minute. Remove the pan from the heat and stir in the cheese and pine nuts. Season to taste with salt. Serve right away or let sit off the heat, partially covered, until ready to serve, and then reheat gently over medium-low heat.