Quick Onion Jam: Place a medium-sized saucepot or skillet over medium heat. Add the butter, and once melted, add the diced onions and a generous pinch of salt. Cook, stirring frequently, until most of the moisture has evaporated and the onions begin to turn lightly golden, about 10–15 minutes. At this stage, add the light brown sugar, water, and balsamic vinegar. Reduce the heat to medium-low and simmer, stirring occasionally, until the liquid has reduced to a syrupy consistency, about 5–10 minutes. Season to taste with salt and reserve until serving.

Mayo: In a small bowl, combine the mayonnaise, parsley, chives, thyme, capers, pickle, lemon zest, lemon juice, horseradish, and garlic. Season to taste with salt and black pepper, then whisk until smooth. Cover and refrigerate until ready to use.

Steak: Let the steaks sit at room temperature for about 30 minutes before cooking to achieve the best sear. Season the steaks generously on all sides with kosher salt. Set them on paper towels while you preheat your pan. Set a large stainless steel skillet over medium-high heat, and once hot, add enough neutral oil to coat the bottom of the pan. Press the steaks into the pan and sear until they form a golden brown crust, about 2-3 minutes. Flip, then lower the heat to medium. Push the steaks to the far side of the pan, then add the butter, garlic, and thyme. Once melted, tilt the pan toward yourself and use a large spoon to gently baste the steaks with the bubbling butter. Cook, continuously basting, for another 1–2 minutes or until you reach your desired internal temperature. For medium-rare, I recommend removing the steaks between 125–130°F (52–54°C) to account for carryover cooking. Transfer the steaks to a wire rack set over a baking sheet and pour the basting liquid over the top. Let rest for a minimum of 5–10 minutes before slicing against the grain into ½” (1.3 cm) slices.

Herb Salad: In a small bowl, toss together the red onion, parsley, chives, and lemon zest.

Assembly: Slice the baguettes crosswise then heat a large skillet over medium heat. Add enough butter to coat the bottom of the pan, then toast the bread cut-side down until golden brown and crispy, about 2-3 minutes. Repeat with the remaining bread. Spread a generous amount of mayo on the toasted bottom slice, followed by a handful of the herb salad. Lay down enough slices of steak to fill the sandwich, then season to taste with flaky salt and black pepper. Spoon on a thin layer of your onion jam, then crown your king with the mayo-slathered top slice. Serve and enjoy!