Place steak in your slow cooker & sprinkle the sliced jalapeno peppers and cilantro over the top
In a medium bowl, combine brown sugar, soy sauce, Worcestershire sauce, vinegar, garlic and 1 Tbsp of the cornstarch. Stir until sugar and cornstarch has mostly dissolved. Pour the sauce mixture over the meat.
Top with mushrooms and onion then sprinkle the salt and pepper over the top.
Cover and cook on low-heat setting for 6-8 hours or on high heat setting for 4-5 hours until steaks are tender.
Mix remaining 2 Tbsp cornstarch with 2 Tbsp of hot water till smooth, then stir in to the sauce.
Let the meat cook an additional 20-30 minutes so the sauce has a chance to just thicken a little.
Serve over mashed potatoes with the sauce spooned over the top and sprinkle with more fresh cilantro if desired.