Heat oil in a large skillet over medium-high heat. Season the outside of roast generously with salt and pepper and brown for 2-3 minutes on each side in the hot skillet, then place roast in a slow cooker sprayed with nonstick cooking spray.
In the same skillet, whisk together tomato sauce, vegetable broth, flour, chili powder, garlic powder and crushed bouillon cubes. Bring to a boil and and cook for 5 minutes, stirring continuously, until thickened, then pour over roast.
Cover and cook on low for 8-10 hours. After cooking, remove beef from slow cooker and shred with 2 forks. Preheat oven to high broil. Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
Divide meat evenly (approx. 1.8 ounces each) between tortillas, top with 2 Tablespoons of cheese and roll up, tucking the sides in, and place seam side down in prepared pan.
Pour sauce from the slow cooker over the top and sprinkle on remaining cheese. Broil for 2-3 minutes, or until cheese is melted and bubbly.