Preheat the oven to 375 degrees . Line a baking sheet with parchment paper. Using a blender, mix the milk, eggs, flour and salt for 1 minute.
In a large nonstick skillet, cook the sausage over medium-high heat; transfer to a bowl. Add 2 teaspoons butter and the onion to the skillet and cook until softened; transfer to the bowl. Wipe out the skillet.
In the skillet, heat 1/2 teaspoon butter over medium heat. Pour 1/3 cup batter into the pan, swirling, and cook until golden. Flip and cook for 1 minute more. Transfer to the prepared baking sheet. Repeat with the remaining butter and batter to make a total of 8 crepes.
Divide the cheese, sausage-onion mixture, cilantro and salsa among 4 of the crepes. Top with the remaining crepes and bake until the cheese has melted, about 5 minutes. Cut into quarters.