Place boneless beef chuck roast in the slow cooker. In a bowl, whisk together beef stock, soy sauce, Thai sweet chili sauce, honey, ginger, garlic, sesame oil and red pepper flakes. Pour over roast. Cook on low for 8-10 hours or high for 4-5 hours.
Once tender, shred the meat. In a small bowl whisk together cornstarch and water. Pour into the slow cooker and stir. Cook another 30 minutes on low until thickened.
To serve: place desired amount of shredded beef onto a lettuce leaf. Top with shredded carrots and sliced cucumbers.