Heat half of the olive oil in a heavy skillet over medium-high heat until shimmering. Add half of your steak strips in a single, flat layer. Sauté for 2 minutes just to form a brown crust on the outside, then transfer to a plate and repeat with the second half of the meat. Pro Tip: Searing all 2 pounds of cold meat at once crowds the pan. This causes the meat to release its juices and boil/steam in its own liquid rather than caramelizing, which ruins the depth of flavor in the sauce.

In a measuring cup, whisk the hot water, beef bouillon cube, and cornstarch together until completely dissolved.

Place the seared beef, chopped onions, minced garlic, soy sauce, brown sugar, Worcestershire sauce, and the diced tomatoes with their juices directly into the crockpot. Pour the cornstarch-bouillon mixture over everything and stir well to lift any heavy starches from the bottom.

Cover and cook on High for 3 hours or Low for 6 hours. During this phase, the connective tissues in the sirloin break down completely into the tomato-soy gravy.

Exactly 1 hour before you plan to serve, lift the lid and fold the raw, sliced bell pepper strips into the hot steak mixture.

Reseal the lid and cook for the final 60 minutes. Crucial: Slow cookers excel at softening food, but dumping the bell peppers in at the very beginning turns them into grey, mushy skins. Adding them in the final hour allows them to steam until tender while retaining their crisp edge and vibrant color.

Turn the crockpot off and let the pepper steak sit uncovered for 5–10 minutes before serving. This cools the starches slightly, allowing the sauce to tighten up into a glossy, thick restaurant-style glaze.