Heat up your grill. Get it as high as you can for charcoal and about 500°F for gas
In a skillet, add olive oil, mushrooms and onions and cook until softened
Add your sausages to the grill and cook until done, turning them every couple minutes
Cut your baguette down the middle. (Toast it on the grill a little if you like)
Layer the sandwich with onions and peppers first, add your mustard and top with the sausages. Cut into four mini sandwiches and serve.