Chop the onion and sauté with butter in a baking dish. Stir in 2 cups of uncooked rice and let it turn to glass. Pour 4 cups of chicken stock.
Dice 100 g Gouda and mix with the rice. Place 450 g spinach in the center, not defrosted. Lid or aluminum foil on top. Bake in the oven at 200 degrees for about 40-45 minutes.