Prep the pork. Pat the pork loin dry with paper towels.
Trim any thick external fat, but keep a thin layer for moisture. Season lightly with salt and pepper on all sides.
Make the sauce. In a bowl, whisk together honey, soy sauce, ketchup, vinegar, garlic, ginger, onion powder, black pepper, and red pepper flakes. It should taste sweet, garlicky, and slightly tangy.
Set up the slow cooker. Place the pork loin in the crockpot.
Pour the sauce evenly over the top. Spoon some under the loin if you like even coverage.
Cook low and slow. Cover and cook on LOW for 4–6 hours (preferred) or HIGH for 2–3 hours. Start checking at the early end of the range.
The pork is done when it reaches 145°F internal temperature and is tender but still sliceable.
Rest the meat. Transfer the pork to a cutting board. Tent with foil and rest for 10 minutes to keep it juicy.
Thicken the sauce. Pour the cooking liquid into a saucepan. Bring to a gentle simmer.
Whisk in the cornstarch slurry and cook 2–3 minutes until glossy and slightly thick. Stir in sesame oil if using. Taste and adjust seasoning.
Slice and sauce. Slice the pork loin into 1/2-inch pieces.
Return slices to the crockpot or a serving platter and spoon the honey garlic sauce over the top.
Garnish and serve. Sprinkle with chopped parsley or green onions. Serve with mashed potatoes, rice, noodles, or steamed veggies to soak up the extra sauce.