To make sirloin steak in a slow cooker, brown the steak in a pan with oil, place the steak on top of the vegetables being used in the slow cooker, cover with broth, and cook for between 3 and 8 hours, depending on the heat setting. Vegetables commonly used include onion, bell pepper and stewed tomatoes. To make gravy, most recipes either thicken the broth in which the meat was cooked or make a rue out of the cooking oil.
Season the sirloin with salt, pepper and garlic powder to taste, and brown both sides of the meat in a skillet over medium heat. Cut the onion and bell peppers into chunks, and place in the bottom of the slow cooker. Place the meat on top of the vegetables, pour over the broth or cooking liquid, and put the lid on. For most meet, cook 3 to 4 hours on the high setting or 6 to 8 hours on low.
To make gravy, remove the meat from the slow cooker about 30 minutes prior to serving, and keep it warm. Mix together water and cornstarch until smooth, then add to the cooking juices. Stir and heat in the slow cooker on high for about 30 minutes, stirring occasionally. When the gravy thickens, serve over the meat.