Step 1: Season the Chicken - Put the chicken pieces in a large bowl and sprinkle them evenly with the kosher salt and black pepper. Give everything a good toss so the seasoning reaches more than just the pieces sitting on top. This first layer of seasoning may seem simple, but it makes a difference because you are flavoring the chicken itself instead of asking the sauce to do all the work later.
Step 2: Coat the Chicken - Sprinkle 2 tablespoons of cornstarch over the seasoned chicken and toss until every piece has a light coating. You are not trying to create a thick breading here. A thin, dusty layer is exactly what you want. It helps the outside of the chicken brown in the skillet and gives the pieces a little more structure as they cook slowly in the sauce.
Step 3: Sear the Chicken - Heat the vegetable oil in a large skillet over medium high heat. Once the oil is hot, add the chicken in a single layer and cook for about 2 to 3 minutes, turning the pieces so they pick up a little color. You are not cooking the chicken through at this point. The goal is simply to get some browning on the outside. If your skillet feels crowded, work in two batches so the chicken sears instead of steaming.
Step 4: Move the Chicken to the Slow Cooker - Transfer the lightly browned chicken to a 6 quart slow cooker. If there are flavorful juices left in the skillet, you can add those too. I try not to leave good flavor behind when it could be heading straight into dinner. Spread the chicken into a fairly even layer so the sauce can surround the pieces once you pour it in.
Step 5: Whisk the Savory Sauce - In a medium bowl, whisk together the low sodium soy sauce, chicken broth, oyster sauce, hoisin sauce, rice vinegar, light brown sugar, sesame oil, minced garlic, ground ginger, and crushed red pepper flakes if you are using them. Keep whisking until the brown sugar is mostly dissolved and everything looks evenly combined. The sauce will seem fairly thin right now, and that is normal. It thickens later.
Step 6: Add the Sauce and Vegetables - Pour the sauce over the chicken and stir gently so the pieces are coated. Add the sliced mushrooms, zucchini, and red bell pepper. If your family likes vegetables with a firmer bite, you can add the mushrooms now and hold the zucchini and bell pepper until the last 45 minutes or so. I usually make that call based on who is sitting at the table because some nights soft vegetables go over better than crisp ones.
Step 7: Slow Cook Until Tender - Cover the slow cooker and cook on LOW for 4 to 5 hours, or until the chicken is fully cooked and tender. Chicken should reach 165°F in the center. Slow cookers can vary, so start checking closer to the 4 hour mark if yours tends to run hot. You want juicy pieces that are easy to bite through, not chicken that has cooked so long it starts drying out around the edges.
Step 8: Mix the Cornstarch Slurry - When the chicken is cooked, whisk the remaining 2 tablespoons of cornstarch with 2 tablespoons of cold water in a small bowl. Stir until the mixture is completely smooth. Cold water matters here because cornstarch can clump when it is added straight to hot liquid. Taking ten seconds to mix the slurry separately gives you a smoother sauce with much less fuss.
Step 9: Thicken the Sauce - Pour the slurry into the slow cooker and stir gently to distribute it throughout the cooking liquid. Cover again and cook for another 20 to 30 minutes. When you come back, the sauce should look thicker, shinier, and much more like something that wants to cling to a scoop of rice. If it still seems a little loose, give it a few more minutes with the lid on.
Step 10: Garnish and Serve - Give everything one final gentle stir, then sprinkle sliced green onions and sesame seeds over the top if you are using them. Spoon the mushroom chicken over hot steamed white rice or chow mein noodles, making sure everybody gets plenty of sauce. Around my table, the sauce is the part people chase around the bowl with the last few bites of rice, so I never mind being generous with it.