In a medium bowl, combine the diced onion, minced garlic, honey, low sodium soy sauce, rice wine vinegar, and grated ginger. Whisk the mixture until it is smooth and well blended, creating a rich and aromatic teriyaki sauce that will coat the beef beautifully. Set this flavorful blend aside for later use in the slow cooker.

Take your beef boneless eye of round roast and slice it lengthwise to decrease its thickness. This ensures even cooking and tender results. Place the sliced beef into the slow cooker, layering it on the bottom, and then pour the prepared teriyaki sauce over the meat, ensuring it is thoroughly covered for maximum flavor infusion.

Cover the slow cooker with its lid and set it to cook on high for 6-7 hours or low for 8-9 hours. The beef will become tender and absorb the delicious teriyaki sauce. As it cooks, delightful aromas will fill your kitchen, promising a mouthwatering meal to come.

Once the cooking time is up, carefully transfer the beef to a cutting board. Using two forks, shred the beef into bite-sized pieces, allowing it to retain some of the teriyaki sauce that has developed. The beef should be fork-tender, making this step easy and enjoyable.

In a small bowl, mix together water and cornstarch until smooth, then add this mixture to the remaining sauce in the slow cooker. Whisk until combined, and cook on high for another 20-30 minutes, or until the sauce thickens to your desired consistency, transforming the dish into a glossy coating for the beef.

Return the shredded beef to the slow cooker and stir it well into the thickened sauce. Ensure that every piece is coated in the irresistible teriyaki sauce. Now, it’s time to serve your Beef Teriyaki Lettuce Cups! Spoon the mixture into crisp lettuce leaves and garnish with chopped green onions and optional sesame seeds for added texture.