Lightly grease a slow cooker. Cut the roast into 3-4 chunks and add to the slow cooker, along with enchilada sauce, bullion granules, garlic, and onion.
Cover and cook on low for 6-8 hours, or until meat is tender. Remove the meat from the slow cooker, leaving the sauce inside, and shred the beef with two forks. Season with salt and pepper to taste. Set aside.
Preheat the oven to 400°F (200°C). Lightly grease a 9×13-inch baking dish. Spread 1/3 cup of the sauce from the slow cooker in the bottom of the dish.
Place 1 1/2 cups of sauce from the slow cooker into a medium saucepan. Whisk in the heavy cream. In a small bowl, whisk together flour and water until smooth. Gradually whisk the flour mixture into the sauce. Bring to a simmer over medium heat. Season with salt and pepper to taste. Simmer while preparing the burritos.
Divide the shredded beef evenly between the tortillas. Roll them up and place them seam side down in the prepared baking dish.
Pour the creamy enchilada sauce over the burritos. Sprinkle with shredded cheese. Bake, uncovered, for about 20 minutes or until the cheese is melted and the sauce starts to bubble.
Garnish with chopped cilantro before serving. Serve with optional sour cream, pico de gallo, or guacamole.