The cut determines how long you need to cook steak in a 150-degrees-Fahrenheit slow cooker. You only have to cook tender cuts to the desired doneness, because if you cook them for a few hours, they attain the same texture as a cheaper cut, with no improvement in taste. So, you should cook fillets and rib eyes for 35 to 45 minutes at 150 F, and tough, cheaper steaks, like flank and sirloin, for two hours.
Heat a couple tablespoons of oil in a saute pan over medium-high heat before or after you slow-cook the steak. Pat the steak dry with paper towels and brown it on both sides, about 1 minute each side.
Add about 1/2 cup water, wine or stock to the slow cooker along with any whole herbs and spices. Add aromatic ingredients and vegetables to the slow cooker, such as onions and garlic, just as you would when braising, if desired. Season the steak on both sides with salt and pepper and add it to the slow cooker, then set the heat to 150 F or low. Slow-cook tender steaks for 45 minutes and cook tough steaks for about 2 hours, or until they reach the desired tenderness.