Update on last night's dinner: it worked! I cooked a small flank steak in the slow-cooker with a few tomatoes, lots of spices (cumin, chili, fajita seasoning mix from Texas), onions, and garlic. The result was the most tender, fall apart beef spiced up all nicely for the fajitas. The tomatoes acted as a nice filler and helped to round out the flavour. I will definitely use this as my stand-by method for beef now.