Dump the package of spinach into a microwavable bowl and microwave it on high for ~4 minutes to defrost it. (You can also let it defrost overnight in the refrigerator.)
Put the defrosted spinach into a colander and press out all the liquid.
Heat the butter or coconut oil over medium heat in a large cast iron skillet and toss in the chopped onions and mushrooms along with some salt and pepper.
Sauté the veggies until the liquid has evaporated and the onions are softened.
Put the ground beef into a large bowl. Set aside.
Throw the coconut milk, parsley, and celery into a container and blend with a hand blender.
Once a puree is formed, pour it over the ground beef and then add the chopped spinach, coconut flour, garlic, measured salt and pepper, fresh nutmeg, beaten eggs, and cooked mushroom mixture.
Gently combine all the ingredients and form the meat mixture into small patties (about 2 inches in diameter). You should have about 30 patties.
Heat 2 tablespoons of coconut oil in a cast iron skillet over medium heat and fry the sliders in three batches.
Cook the sliders for 3 minutes on each side.
Once they're done cooking, place the patties on a plate and sprinkle with fleur de sel. Dig in with some heated marinara sauce!