In a small bowl, mix the softened butter with crushed garlic and finely diced fresh dill. Stir until the mixture reaches a smooth, even consistency. Set aside for later use.

Using a sharp knife, slice the steak into thin, even strips. Tip: For easier slicing, place the steak in the freezer for a few minutes before cutting.

Heat a skillet over high heat and add a drizzle of oil. Once the oil is shimmering, add the sliced steak to the hot pan. Generously season with salt and cook until the beef is seared to a golden-brown hue. Remove from the skillet and set aside.

Slice the onion into thin half-rings. Using the same skillet, sauté the onions over medium heat until they turn golden brown and caramelized. Remove from heat and set aside.

Cut the baguette in half lengthwise. Evenly spread the prepared herb butter on both sides of the bread. Layer the steak slices over the bottom half of the baguette, followed by the caramelized onions. Finish with slices of Havarti cheese, ensuring the sandwich is fully covered.

Place the open sandwich under the broiler for about 2 minutes, or until the cheese is melted and bubbling.

Serve immediately while warm, and enjoy the rich flavors and textures of this hearty steak sandwich.