Before starting, preheat the oven to 450°F.Wash and dry all produce. Cut tomatoes into 1/4-inch pieces.Add mayo, mustard and horseradish sauce 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 (1 tbsp) oil, then onions. Cook, stirring occasionally, until slightly softened, 3-4 min. Add garlic puree and 1 tsp (2 tsp) sugar. Season with salt. Cook, stirring occasionally, until onions are dark golden-brown, 3-4 min. Remove from 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 (2 tbsp) oil, 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 buns so they don't burn!)

Add 1 tbsp (2 tbsp) oil, 1/4 tsp (1/2 tsp) sugar 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 mustard-horseradish sauce over bottom buns, then stack with steaks and caramelized onions. Close with top buns.Divide steak sammies and salad between plates.