Melt the coconut oil in a large skillet over medium heat, and saute the red onion until it starts to become translucent, about 5 minutes. Add in the mushrooms and salt and saute until tender, another 6 to 8 minutes. Add in the vinegar and saute another minute. Turn off the heat and stir in the olive oil.
Place the spinach in a large bowl and pour the hot vegetables over the top, tossing well until the leaves are glossy and slightly wilted. Plate the salad and top with crumbled goat cheese and freshly ground black pepper. Serve warm!