In a mixing bowl, season potatoes with salt and pepper and toss with oil to coat. Place on a baking sheet and roast in the oven for 20 minutes. Remove briefly from oven and toss in onions, and return to oven to roast until potatoes are tender, approximately another 20 minutes. Transfer roasted potato mixture to a bowl. Drizzle honey over potatoes, add butter, mixing well, and then gently toss in spinach. The heat from the potatoes will wilt the spinach.