Start by slicing the cucumbers and red onion thinly. Place them in a bowl and add vinegar, sweetener, salt, and pepper. Toss everything together until well combined. Let the salad sit while you cook the beef so the flavors can blend.
Heat sesame oil in a large skillet over medium-high heat. Add the ground beef and break it apart while cooking. Continue stirring until the beef is browned and fully cooked. Drain excess fat if needed to keep the dish balanced.
Add minced garlic and cook until fragrant. Stir in soy sauce or coconut aminos, sweetener, rice vinegar, ginger, and red pepper flakes. Let the mixture simmer for a few minutes so the sauce thickens and coats the beef evenly.
Remove the skillet from heat and sprinkle in green onions and sesame seeds. Serve the spicy beef alongside the chilled cucumber salad. Enjoy immediately for the best flavor and texture contrast.