For the teriyaki sauce, in a small bowl whisk together the soy sauce, mirin, sugar, sesame oil, ginger, garlic and water.

For the steak tips, heat 2 tablespoons of oil in a 12 inch cast iron skillet over medium-high heat. When the oil is hot, add half of the beef cubes and season them with salt and pepper. Allow them to cook undisturbed for a couple of minutes, then stir, cooking just long enough to sear the beef without cooking it through, about 3-4 minutes. Use a slotted spoon to transfer the browned beef to the slow cooker. Add the remaining 2 tablespoons of oil to the pan and repeat with the remaining beef cubes.

Add the teriyaki sauce mixture to the slow cooker with the beef. Layer the onion and bell peppers over the top and with out stirring, cover and cook for 6-7 hours on low or 3-4 hours on high, until fork tender. Add the mushrooms, cover and continue to cook 20-30 minutes on low, just until the mushrooms have softened.

Combine the water and cornstarch in a small bowl and add the mixture to the slow cooker, stirring for several minutes until the sauce has thickened. Sprinkle on the green onions and sesame seeds and serve over the rice.