Sauté onion and mushrooms in butter in a large frying pan over medium-high heat until softened and starting to brown, about 4 minutes. Remove from heat. Add salt, honey, mustard, and mirin to warm pan, whisking to combine. Slowly whisk in oil. Set aside.
Put spinach in a large bowl. Pour dressing on top and toss gently. Sprinkle with walnuts.