Lightly spray a 10x15" jelly roll pan. Line with parchment paper and spray again.
In a saucepan melt butter.
Add flour and salt and stir until smooth.
Gradually add milk and cook, stirring, until thick.
Add a small amount of hot sauce to egg yolks, whisk and add to the sauce. Cook and stir for 1 minute more.
Remove from heat, add sugar and nutmeg.
Beat egg whites until stiff, but still shiny. Fold into mixture.
Spread batter evenly in prepared pan and bake in a preheated 350°F oven for 45 minutes.
For Mushroom filling, saute onions in oil until translucent. Put mushrooms in a pot over moderate heat. Let the liquid cook away and mushroom just about stick to the pot, add the onions. Add the remaining ingredients, blending well.
Turn out the baked souffle on 2 overlapping pieces of parchment paper (make sure the paper is bigger than the pan), lift off the pan and gently remove the parchment paper. Spread the filling and roll, using the under paper to help you.
If you want to refrigerate, slightly spray aluminum foil with Pam, and wrap the roll. Reheat in a 350°F oven for 15 minutes.