Season beef with salt and pepper, sprinkle with flour and shake off excess.

Add olive oil to pan and sear beef on all sides for 2 to 3 minutes a side in a heavy skillet. Set aside.

Add butter to pan and then cook mushrooms until they start to release their juices (3-4 minutes).

Stir in onion and cook until translucent. Add minced garlic and cook for another minute.

Stir in flour to combine and then add tomato paste and stir to coat well.

Slowly add the chicken stock, stirring to combine and scraping up any brown bits from bottom of pan.

Bring to a simmer and let thicken slightly before removing from heat. Add San Marzano tomatoes and combine well.

Line bottom of slow cooker with carrots, parsnips, celery.

Place roast on top, then place tomato, mushroom and onion mixture over top of that.

Cook on low for 6-7 hours or until meat is tender.

Serve with mashed potatoes.