If needed, cut roast to fit into a 3 1/2- to 6-quart slow cooker. Spray slow cooker with cooking spray.
Place the onion, garlic, carrot and celery in the slow cooker. Sprinkle flour over vegetables and mix.
Add olive oil to large skillet over medium heat and brown roast on all sides. (See Note 1)
In a bowl whisk together the beef broth, tomato paste, mustard, herbs, salt and pepper. Place roast on top of vegetables. Deglaze skillet with red wine (scraping up the bits of caramelized bits on bottom of pan) and pour wine and marinade liquid over roast.
Cover and cook on low-heat setting for 10 to 12 hours or high-heat setting for 5 to 6 hours. Season to taste with salt and pepper.
Remove roast from slow cooker and set aside, covered. Discard bay leaves. Scoop out half of the vegetables and 2 cups of cooking liquid and purée for 30 seconds in a blender.
Slice or break up pot roast into pieces and place back in the slow cooker and pour sauce over meat and cover to stay warm or serve immediately with sauce on the side. Garnish with chopped parsley.
If not browning the meat add the red wine to the next step and top vegetables with meat.