Step 1: Sear the beef - Heat the oil in a skillet over medium high heat. Add the thinly sliced flank steak and quickly sear it just until the outside is browned. You are not cooking it all the way through here. You are just giving it a little color and flavor to build on later.
Step 2: Add the beef to the slow cooker - Transfer the seared beef into the slow cooker. Make sure you scrape any flavorful bits from the pan into the cooker too. That is good stuff.
Step 3: Build the sauce and base - Add the onion, beef broth, soy sauce, brown sugar, ground ginger, black pepper, garlic, shredded carrots, and sliced bell peppers to the slow cooker. Give everything a gentle stir so the beef is coated and the vegetables are tucked into the liquid.
Step 4: Slow cook until tender - Cover and cook on low for four to five hours. The beef will become tender and soak up all that savory flavor. This is the part where your house starts to smell like dinner and everyone wanders into the kitchen asking when it will be ready.
Step 5: Add the quick cooking vegetables - Stir in the broccoli florets and snow peas. These go in near the end so they stay bright and crisp tender instead of turning mushy.
Step 6: Thicken the sauce - Mix the cornstarch with half a tablespoon of cold water until smooth. Stir this mixture into the slow cooker. This helps the sauce thicken up so it coats the noodles instead of pooling at the bottom.
Step 7: Add the noodles - Add the cooked and drained noodles to the slow cooker. Gently toss everything together so the noodles get coated in that savory sauce without breaking apart.
Step 8: Heat through and serve - Let everything heat through for about fifteen to twenty minutes. Taste and adjust seasoning if needed. Serve warm and top with scallions and sesame seeds if you like a little extra flair.