Add the onion, garlic, and gnocchi onto a large sheet pan. Tear the sausage meat into about 1-inch pieces and add to the sheet pan. Season with salt and pepper along with the crushed red pepper flakes and nutmeg. Zest over the lemons then drizzle with the olive oil. Toss well to combine and arrange in a single layer.
Cut the lemons in half and add to the pan, cut side down. Roast the gnocchi for 30 to 35 minutes or until the sausage and gnocchi are cooked and golden brown, tossing once halfway through.
Remove the sheet pan from the oven, remove the garlic cloves and lemon halves, then immediately add on the spinach and cover with foil or an upside-down sheet pan to trap in the heat. Set aside for 5 minutes to steam the spinach.
Meanwhile, squeeze the roasted garlic from its skins and add to a small bowl. Juice over the roasted lemons and, using a fork, mash and mix together the garlic and lemon juice. Uncover the pan and pour over the garlicy mixture, tossing well to evenly coat and mix.
