Bring the vinegar, water, salt, and sweetener to a boil in a small pot (or microwave in a glass bowl). Transfer to a glass bowl and add the red onions, stirring to coat completely. Let sit for 30 minutes while you prepare everything else.

Dry the beef well and season to taste with salt. Sprinkle the baking evenly over the steak and rub to distribute it evenly.

Preheat the oven’s broiler and oil a rimmed baking sheet with avocado oil to prevent sticking.

Place the steak onto the baking sheet. Toss the tomatoes with 1/2 tablespoon avocado oil and place around the steak. Broil on high for 8-10 minutes, for medium rare (125ºF), or until cooked to your liking. Let the steak rest for 10 minutes before slicing. If the tomatoes are not lightly charred yet, return them to the oven and broil until burst and lightly charred.

While the steak is cooking, prepare the dressing by whisking together all the dressing ingredients. Set aside.

To serve, drain the onions, combine with the spinach, tomatoes, and dress with the reserved horseradish. Top with sliced steak and serve.