In a large glass bowl, whisk your coconut milk, Dijon mustard, flour and a pinch of sea salt; whisk until really well combined.
Heat a skillet large with the oil over medium-high heat.
Add in your onions and garlic, sauté for few minutes until just beginning to soften.
Stir in your mushrooms, and continue cooking for 4-5 minutes more, stirring frequently until golden brown. Season with a tiny pinch of sea salt and pepper.
Squeeze in your fresh lemon juice and stir well.
Reduce the heat to a low, and pour in the coconut milk mixture, then simmer until just thickened, about 4 minutes or so.
Quickly add in the fresh spinach and stir until just wilted, just a minute or two.
Enjoy warm!