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 the 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 2 minutes per side for medium. Don’t overcook, or the steak will be dry and chewy.
Remove the cooked steak cubes from the skillet and arrange them on top of the salad. Whisk the vinaigrette one more time and drizzle it on top. Serve immediately.
*Sirloin is very flavorful, but it can be a bit tough. You can tenderize it overnight in a vinegar-based marinade prior to sauteing. Or choose a more tender cut such as tenderloin.