Slice each piece of the baguette in half, you should have enough for 3 big sandwiches. Set the pieces aside.
In a small blender add the red peppers, calabrian chili paste and a shallow 1/4 cup of water. Blend until you have a smooth sauce. Season with kosher salt and black pepper to taste.
Thinly slice the onion and place a large sauté pan over medium heat. Melt the butter in the pan, then add a tablespoon of olive oil. Add the onion and season with a pinch of salt. Cook, stirring occasionally, to let the onion *almost* caramelize. True caramelization takes up to 40-45 minutes, but in the interest of time, I cooked mine for about 20-25, letting the onions get soft, jammy, and lightly brown.
When the onions are almost done, drizzle the balsamic glaze over them. Give everything a quick mix in the pan, then transfer to a small bowl.
Cut the broccoli rabe in half, then into long strips, slicing through the center of the stem pieces. Add the broccoli rabe to the same pan you cooked the onions, and top them with a drizzle of olive oil and pinch of salt.
Shake the broccoli rabe around the pan, letting it charr in some places. Cook for about 3 minutes on each side before removing from the pan.
Assemble each sandwich with the red pepper sauce first, then a layer of broccoli rabe, then slices of smoked mozzarella. Broil this in an oven or toaster oven for 5 minutes, or until the cheese is melty. Add the onions to the other slice of the baguette, smash together, and eat.