Put all ingredients in a blender and process for 30 seconds. Stop and scrape down sides, blend for another 15-20 seconds.
Place batter in refrigerator for 1-2 hours. Batter can be made the day before and stored in the refrigerator overnight. Pour approx. ¼ cup batter in a heated crepe pan. Watch for bubbles to form and carefully lift at the crepe edge and flip to brown the opposite side. Crepes can be made a day ahead and stored in the refrigerator or can be stored in the freezer in tightly sealed freezer bags for up to 4 months.
Melt butter in pan over medium heat, incorporate the flour slowly, whisking until blended. Add chicken stock, continuing to whisk until smooth. Add additional liquid as needed to create a creamy, smooth sauce. Reserve ½ cup of the sauce for topping the crepes.
To the rest of the sauce add the cooked chicken, asparagus and diced mushrooms. Season to taste and set aside to cool.
Add the sliced mushrooms and shredded cheese to the ½ cup of sauce reserved for the topping.
To assemble the crepes, take approx. 1/3 cup of the filling per crepe, spooning the mixture in a band from one edge to the other (not spreading it over the entire crepe surface). Roll each crepe and place seam side down into a casserole dish. Top the assembled crepes with the mushroom/cheese sauce and place the casserole dish uncovered in a 325 degree oven for approximately 20 minutes.