Combine the 4 cups spinach, 2 cups escarole, 6 sliced mushrooms, and stalk of sliced celery in a large salad bowl.
Combine 1/3 cup oil, 2 tablespoons lemon juice, 1 clove minced garlic, 1 teaspoon mustard and a good-sized pinch of kosher salt and freshly ground black pepper in a glass jar with a tight-fitting lid. Shake vigorously to combine.
Drizzle half the dressing over the salad, toss and add more if necessary. Taste and season with more salt and pepper if desired.
Serve immediately. If making ahead, keep the salad and the dressing separate until right before serving.