In a large bowl, whisk together soy sauce, beef broth, brown sugar, garlic, ginger, sesame oil and red pepper flakes, if using.
Place short ribs into a 6-qt slow cooker. Stir in soy sauce mixture until well combined.
Cover and cook on low heat for 7-8 hours or high heat for 3-4 hours.*
In a small bowl, whisk together cornstarch and 1/4 cup water. Stir in mixture into the slow cooker. Cover and cook on high heat for an additional 30 minutes, or until the sauce has thickened.
Serve immediately, garnished with parsley and sesame seeds, if desired.
Notes
Use the right size crockpot. A 6-qt slow cooker is ideal for this recipe. If the slow cooker is too small, the short ribs may not cook evenly. If the slow cooker is too large, the liquid may cook down too quickly, causing the short ribs to burn.
Make ahead of time. Short ribs taste even better the very next day, perfect to make this ahead of time 1 day in advance (scraping off the fat) and reheating in the oven until warmed through.
Use reduced sodium soy sauce. Reduced sodium has less sodium and less salt without compromising flavor.
Mix cornstarch with water. Combining the cornstarch with water first (also known as a slurry) will prevent lumps in the sauce, ensuring a smooth consistency and even thickening.
Adjust the consistency. Add in the slurry slowly in a fine stream, stirring constantly. If the sauce is too thick, add a splash of water or beef broth until the desired consistency is reached.
Test for doneness. The meat should slide out freely from the bone without resistance.
Serve over rice. This is best served over rice (or your favorite starch such as mashed potatoes or cauliflower rice) to sop up all the leftover cooking juices.
Reheat in the oven. Leftover short ribs can be rubbery when reheated. For best results with leftovers, reheat in the oven at 350°F, covered in aluminum foil until warmed through, about 15-20 minutes.
Freeze leftovers. Let cool completely; portion into plastic freezer bags in individual servings with leftover cooking liquid (to keep the short ribs moist during freezing and reheating), squeezing out any excess air before sealing. If leftovers are going to be used as shredded meat, shred first, then freeze. To reheat, thaw overnight in the fridge, reheating in a low oven to prevent from drying out at 250°F, covered in aluminum foil until warmed through, about 20-30 minutes.