For the crepes, mix flour and salt, whisk in the eggs, and then add the milk and oil. Cook thin crepes in a relatively large pan. They should not be crispy, so do not cook them for too long.For the filling, finely chop the shallots and place them in a sauté pan with the butter over relatively low heat. Let them simmer gently until they are completely soft. Stir them occasionally.Meanwhile, clean and finely chop the mushrooms. You can also chop them in a food processor. Add the mushrooms to the shallots and let everything simmer gently until all the liquid from the mushrooms has evaporated. Stir the mixture regularly.Then add 2 tablespoons of crème fraiche to the mixture and season with salt and pepper.Let the duxelles cool slightly.Place a crepe on the cutting board and place a slice of ham on top. Add a couple of large spoonfuls of the mushroom duxelles on top, then roll up the crepe as you would a spring roll.Repeat the process until you have 4-6 filled crepes.Place them in oval, ovenproof individual dishes (or a larger ovenproof dish). Distribute a good spoonful of crème fraiche over each crepe and sprinkle with grated Gruyere. Gratinate in a 240-degree hot oven for 15 minutes until the surface is golden.Serve immediately with a good mixed salad on the side.