In a medium bowl, combine 4 tablespoons of softened unsalted butter, 2 minced garlic cloves, and 2 tablespoons of freshly chopped dill. Mix until smooth and well combined.

Freeze ribeye steak for about 15 minutes until slightly firm, then slice into thin strips against the grain, aiming for 1/4-inch thickness.

Heat a skillet over high heat, adding a drizzle of olive oil until shimmering. Season sliced steak with salt, then add to skillet, cooking for 2-3 minutes per side until golden brown.

In the same skillet, lower heat to medium and add thinly sliced yellow onions. Cook for 8-10 minutes, stirring occasionally, until soft and golden brown.

Slice baguette lengthwise and spread herb butter on both cut sides. Layer steak strips on the bottom half, followed by caramelized onions, and top with havarti cheese.

Preheat the broiler to high and place open-faced sandwiches on a baking sheet. Broil for about 2-3 minutes until cheese is melted and bubbly.

Remove sandwiches from the oven, let cool slightly, slice into portions, and serve warm.