Heat 2 TBS butter in a non-stick pan over medium-low. Once melted add the onions and cook stirring occasionally until golden brown and caramelized. About 45 mins.

To make the aioli: combine all the ingredients together in a bowl. Stir until well combined. Let rest for 15 minutes.

To shape the patties: allow the beef to come to room temperature. Form roughly the size of a 2 inch meatball using your hands to roll them out. It should yield between 8-10. Place between two sheets of parchment paper and use the back of a small side plate to press down each meatball into a smashed patty.

Depending on the shape of the bread used, you may need to use 2 patties per sandwich, which is what is pictured here. If you've prepared more than needed, store each patty between parchment paper and freeze for the future. Use to make smashed burgers or repeat this recipe.

Heat a griddle pan or use the same pan for the onions on medium heat. Cook the patties on both sides until browned and no longer pink. Season both sides with some salt and pepper. Top each patty with some caramelized onions during the last few seconds of cooking to reheat them.

Wipe out the pan or griddle and return to medium heat. For this you can use tongs and scrape the fat with paper towel since it will be hot to the touch. Butter one side of each bread and place that side down to cook. We won't be flipping the sandwich like you would a grilled cheese. So ensure that all the pieces of bread you will use to make a sandwich are toasting all at once.

On the insides of the bread, spread some mustard on one side and the horseradish on the other bread. Next add the patties with the onions to the pieces spread with horseradish and build your sandwich on there. Finish with a handful of greens. Once the bread slices are perfectly golden, take the top pieces of bread (the one with the mustard) and sandwich that on top. Use a large spatula or back of a plate/pan to press that down and seal everything in place.

Slice in half and serve immediately.