Before starting, preheat the oven to 450°F. Wash and dry all produce. Cut tomatoes into 1/4-inch pieces. Peel, then cut onion into 1/4-inch slices. Add mayo, mustard and horseradish to a small bowl. Season with salt and pepper, then stir to combine. Set aside. Pat steaks dry with paper towels, then season with salt and pepper.

Heat a large non-stick pan over medium heat. When hot, add 1/2 tbsp oil (dbl for 4 ppl), then onions. Cook, stirring occasionally, until slightly softened, 3-4 min. Add garlic puree and 1 tsp sugar (dbl for 4 ppl). Season with salt. Cook, stirring occasionally, until onions are dark golden-brown, 3-4 min. Remove the pan from the heat, then stir in half the vinegar. Transfer onions to a plate. Set aside.

Heat the same pan over medium-high. When hot, add 1 tbsp oil (dbl for 4 ppl), then steaks. Pan-fry until cooked to desired doneness, 3-4 min per side.**

Meanwhile, halve buns, then arrange on an unlined baking sheet, cut-side up. Toast buns in the top of the oven until golden, 2-3 min. (TIP: Keep an eye on the buns so they don't burn!)

Add 1 tbsp oil, 1/4 tsp sugar (dbl both for 4 ppl) and remaining vinegar to a large bowl. Season with salt and pepper, then whisk to combine. Add spring mix and tomatoes, then toss to combine.

Thinly slice steaks. Spread horseradish sauce over bottom buns, then stack with steak and caramelized onions. Close with top buns. Divide steak sammies and salad between plates.