Step 1: Season the Steak - Start by placing your bite sized steak pieces in a bowl. Sprinkle over the salt and black pepper and give everything a gentle toss so each piece is evenly coated.
Step 2: Sear for Flavor - Heat the oil in a skillet over medium high heat. Add the steak pieces in batches so you do not overcrowd the pan. Let them sear for a minute or two until lightly browned on the outside. You are not cooking them through here, just building flavor.
Step 3: Transfer to Slow Cooker - Move the seared steak pieces into your slow cooker. This is where everything is going to come together.
Step 4: Add Pineapple - Scatter the drained pineapple chunks over the steak. This is where that sweet brightness starts to come in.
Step 5: Make the Sauce - In a bowl, whisk together the pineapple juice, teriyaki sauce, soy sauce, brown sugar, minced garlic, and cornstarch. Make sure the cornstarch is fully mixed in so the sauce thickens smoothly later.
Step 6: Pour and Coat - Pour the sauce over the steak and pineapple. Give it a gentle stir so everything is coated evenly.
Step 7: Slow Cook to Perfection - Cover and cook on low for 4 to 5 hours. As it cooks, the steak becomes tender and the sauce thickens into a rich, glossy coating.
Step 8: Stir and Serve - Before serving, give everything a good stir to bring that sauce together. Spoon it over rice and finish with scallions if you like.