Sauté the sliced onion and five cloves worth of the minced garlic over medium heat in 8 tablespoons of the butter until reduced and beginning to brown.
Add the spinach, beans, seasonings and 3 cups water. Cover and roast in the oven for 2 hours. Check on it half way and give it a stir.
After 2 hours, take the lid off and turn the heat up to 200 C / 400 F. Bake another 10-15 minutes until reduced and slightly browned.
Remove from oven and stir in the remaining butter and minced garlic. Taste and adjust salt to your liking (I added an additional 0.5 teaspoon and served with a sprinkle of salt on top).
Notes
you want a little bit of browning on the onions and garlic to get some of that complex flavor development.