In a large bowl, whisk together flour, eggs, milk, mayonnaise and salt until smooth. If there are lumps pour through a sieve. Cover the bowl with plastic wrap and place the batter in the refrigerator for at least 30 minutes. The batter will keep in the refrigerator for up to 48 hours.
Heat a 10-inch nonstick skillet over medium to high heat. Grease the pan with butter. Scoop 3 tablespoons of batter onto the skillet, swirling the batter to create an even surface.
Cook the crepe for about 2 minutes, until the bottom is light brown. With a spatula, flip and cook the other side for another minute. Keep warm in the oven, with sheets of parchment paper in between each crepe, until serving.
Make the savory crepe: In a cast-iron skillet, melt butter. Saute onion with salt for 3 to 5 minutes. Add mushrooms and saute until soft, another 5 to 7 minutes.
Toss in spinach and stir until wilted. Break cream cheese up into pieces and stir until melted. Sprinkle with pepper. Turn off heat. Let cool slightly. Fill crepe and fold over.
Make the sweet crepe: In a medium saucepan over medium heat, combine cocoa, sugar and water. Bring to a boil and let boil 1 minute. Remove from heat. Let cool slightly.
On a plate, lay crepe flat. Fill with strawberries, then roll up and drizzle with chocolate sauce.