Heat 2 tablespoons oil in a large skillet over med/high heat.
Saute the tenderloin chunks quickly for about 5 minutes(watching them closely to not burn the outsides of the meat) to get some color on them. This will get rid of a little fat and liquid from the meat as well.
With a slotted spoon, scoop out the meat and place in your slow cooker, discard any fat or liquid from the meat cooking.
Add the mushrooms, leeks, carrots, water chestnuts, and garlic to your slow cooker also.
In a medium sized bowl, whisk together the orange marmalade, chili sauce, hoisin sauce, dry sherry, soy sauce, and crushed red pepper.
Pour sauce mixture over the ingredients in the slow cooker, and stir to coat everything well.
Cover with lid, put your slow cooker on High and cook for 2 1/2 hours.
After 2 1/2 hours, mix the cornstarch and water in a small cup until smooth, and add it to the mixture in your slow cooker.
Stir for a couple minutes, until you see the sauce thicken slightly, then close lid again and let cook on High for another 30 minutes.