To make the sauce combine all ingredients in a bowl. Set aside in refrigerator.
In a medium saute pan over medium-low heat, add butter. Once butter melts, add the onions, stir occasionally until onions are soft and caramelized. About 15-20 minutes. Season with salt and pepper. Remove from pan.
Using another saute pan, spray with cooking spray, split the sausage and place in pan, brown on both sides until heated through.
Cut the baguette into 4 portions, split and spread top and bottom generously with sauce. Spread ¼ cup of spinach and onions on the roll. Sprinkle cheddar cheese and place the sausage on top.
Enjoy with your favorite side: salad, slaw, fries or kettle chips.