Cut the steak diagonally across grain into 1/4 inch thick slices.
Place steak into the bottom of the slow cooker. Sprinkle salt, pepper, and dried dill weed onto the steak.
In a small microwave-safe bowl, add the chopped onion, garlic, and olive oil. Stir the onion and garlic until coated with the oil. Microwave on high for 3-5 minutes or until translucent, fragrant, and garlic looks slightly golden.
In the bowl with onion and garlic, stir in one cup of the beef broth, Dijon mustard, and Worcestershire sauce.
Pour the broth mixture over the beef in the slow cooker.
Add the mushrooms to the top.
Place the lid tightly on the slow cooker and set to cook on high 2-3 hours or low for 5-7 hours.
I recommend cooking on low if possible because sirloin is a leaner cut of meat and could dry out on high heat.
Use a small whisk or fork to whisk the cornstarch into the remaining ½ cup of beef broth to make a slurry.
Stir the slurry to the juices in the slow cooker.
Place the lid back on and set it to cook for another 10-15 minutes. The sauce will thicken during this time.
Stir in the sour cream, place the lid back on, and let it cook for about 5 minutes. At this point, the sauce will be hot, thick, silk, and delicious.
Please give it a taste for salt and then serve over egg noodles. If you want, sprinkle with fresh parsley or thyme.