In a medium bowl, combine the thinly sliced cucumbers and red onion. Pour in the apple cider vinegar, add the additional sugar-free sweetener, salt, and black pepper. Toss the mixture well until all the vegetables are coated, then set the salad aside to marinate for at least 10 minutes.

Heat a large skillet over medium-high heat and add the ground beef. Cook the beef for about 5-7 minutes, breaking it apart with a spatula until it is fully browned and cooked through.

Once the beef is browned, reduce the heat to medium. Add the minced garlic and sesame oil into the skillet, sautéing for 1-2 minutes until fragrant.

Stir in the soy sauce (or coconut aminos), sugar-free sweetener, rice vinegar, crushed red pepper flakes, and ground ginger. Cook this mixture for an additional 2-3 minutes, stirring frequently.

Remove the skillet from heat and stir in half of the sliced green onions and sesame seeds, mixing thoroughly.

Plate the spicy keto Korean ground beef and garnish with the remaining green onions and sesame seeds. Serve alongside the refreshing marinated cucumber salad.