Prepare the Onions: In a cast-iron skillet, melt the butter over medium heat. Add the sliced red and yellow onions and sauté for 15 minutes until softened. Lower the heat and season with 1 teaspoon of salt. Remove the onions from the skillet and set aside.
Cook the Steak: Season both sides of the steak with 1 tablespoon of salt and 1 tablespoon of pepper. Place the seasoned steak in the same skillet and sear for 5-6 minutes per side. Remove the steak from the pan and let it rest.
Assemble the Sandwich: Preheat your oven to 375°F (190°C). Lay the two halves of the French bread open. Spread a generous amount of jalapeno pesto on both sides. Divide the sautéed onions evenly and layer them on each half of the bread. Slice the rested beef and arrange it on one side of the bread. Top the beef with the Muenster cheese.
Bake and Serve: Place the open sandwich into the preheated oven and bake until the cheese is melted and the bread is toasted. Remove from the oven and let sit for a minute. Fold the two halves of the sandwich together, slice, and serve immediately.