In a mixing bowl, combine flour, eggs, milk, water, melted butter, thyme, sugar (if using), salt, and black pepper. Whisk all ingredients together until the batter is smooth and free of lumps. Allow the batter to rest for 30 minutes at room temperature.

Heat a tablespoon of oil in a skillet over medium heat. Add the chopped shallots and minced garlic, sautéing until translucent, about 3-4 minutes. Incorporate the thyme and sliced mushrooms, cooking until tender and lightly browned, about 5-7 minutes. Finally, stir in the spinach and continue to cook until wilted, about 1-2 minutes.

In a non-stick skillet, melt a small amount of butter over medium heat. Pour approximately 1/4 cup of the prepared batter into the skillet, tilting it to evenly coat the surface. Cook until the edges start to lift and the bottom is lightly golden, about 2 minutes. Carefully flip the crepe and cook for an additional minute until golden.

To assemble your Mushroom Crepes, place a generous spoonful of the filling in the center of each crepe. Fold or roll them to enclose the filling and then serve warm.