Chop the potatoes into small pieces and boil them in a pot of water for about 20 minutes or until you can easily poke a fork through them. No need to peel the potatoes first – the skins have most of the nutrients.
In a small pan, melt the plant-based butter and add in the onion. Stirring frequently, cook for about 5 minutes until the onion softens and starts to brown. Then add the garlic and cook for 2-3 minutes more.
Add 1/3 cup of the potato boiling water to the onion-garlic mix, along with the plant-based milk. Remove from heat and set aside.
Drain the potatoes, transfer them to a large bowl, add the onion-milk mixture, and mash it all together with a fork, potato masher, or mixer. Add in the spinach and salt and pepper to taste. Mix well and serve hot, along with whatever else is for dinner. Enjoy!