In a small bowl, whisk together the vinegar, garlic, Dijon mustard, salt, and pepper. Add the olive oil in a slow, steady stream, whisking constantly until the dressing is emulsified. Set aside.
Prepare the salad:
Arrange the mixed greens and tomatoes on a large serving platter. Set aside.
Cook the steak:
In a large bowl, mix together the kosher salt, black pepper, paprika, garlic powder, onion powder, and dried thyme. Add the steak cubes and toss to coat.
In a large skillet, heat the olive oil over medium-high heat. Add the seasoned steak cubes. Cook until done to your liking. I suggest cooking for 2 minutes per side for medium-rare. Don’t overcook, or the steak will be dry and chewy.
Remove the cooked steak cubes from the skillet and arrange them on the salad. Whisk the vinaigrette one more time and drizzle it on top. Serve immediately.
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Notes
*Make sure to use top sirloin, not bottom sirloin, which is a tougher cut. Even top sirloin can become a bit tough, especially when overcooked. You can tenderize it overnight in a vinegar-based marinade or choose a more tender cut, such as tenderloin. Storing leftovers: You won't be able to keep the dressed salad greens; they will wilt. But you can keep the cooked steak in the fridge, in an airtight container, for up to 4 days. Reheat the leftovers gently, covered, in the microwave at 50% power, or enjoy them cold like cold cuts.The CDC recommends cooking meat, poultry, and eggs thoroughly.