If you're making it in the oven, preheat oven to 275 degrees F. If making it in a slow cooker, no preheating required.
Season roast with salt and pepper, then sprinkle flour on both sides.
In large cast iron dutch oven, heat olive oil over medium high heat.
When hot, add roast and sear on both sides until a crisp brown crust forms.
Remove roast from pot and set on a place.
Add onion, carrots, and celery to pot, and toss with oil left in pan. Add another tablespoon of oil if you feel it's needed. Season with salt.
Add garlic, toss around, and cook for 2 minutes.
Add beer, scraping the drippings from the bottom of the pot, then add beef broth and rosemary, stirring everything.
At this point, if you're going to cook in a slow cooker, place roast in slow cooker and pour vegetable mixture and all liquid over roast. Cook on low for 8-10 hours or high for 4-6 hours, until meat easily shreds.
If you're going to continue cooking in dutch oven, return roast to pot. Cover, then place in 275 degree oven and cook for 4 hours, until meat easily shreds.
Once meat is tender from either cooking method, remove meat from slow cooker or dutch oven and place in large bowl. Shred the meat with two forks.
Add dijon mustard to broth and vegetable mixture.
Using an immersion blender, puree vegetable and beer mixture.
Return shredded meat to slow cooker or dutch oven and stir to mix.
Pile beef and sauce on sandwich bun, on top of mashed potatoes, or noodles.
Top with chipotle cream cheese