In a large bowl, whisk together the flour, milk, eggs, melted butter (or oil), salt, and black pepper until smooth. Let the batter rest for about 30 minutes to allow the gluten to relax.
In a small pan, sauté the chopped onions and bell peppers over medium heat until softened, about 5 minutes. Add the chopped spinach or kale and cook for an additional 2 minutes. Remove from heat and let cool.
Heat a non-stick skillet or crepe pan over medium heat. Lightly grease with butter or oil.
Pour a small amount of batter into the pan, swirling to coat the bottom evenly. Cook for about 1-2 minutes, or until the edges start to lift and the bottom is lightly golden.
Flip the crepe and cook for an additional 1 minute on the other side. Transfer to a plate and cover to keep warm. Repeat with the remaining batter.
Once the crepes are cooked, spread a small amount of the sautéed vegetable mixture and cheese (if using) on one half of each crepe. Fold or roll the crepes to enclose the filling.
Garnish with fresh herbs if desired. Serve the savory crepes warm, as an appetizer or main dish.
Enjoy your meal!