Beef Pot-Roast with Guinness

I made this in early January, when the weather was super crappy.  OMG it was amazing.

2 tbsp vegetable oil
2 lb top round beef roast
1 onion, roughly chopped
4 celery sticks, thickly sliced
4 pototoes, large chunks
3 carrots, thickly sliced
2 tbsp flour
2 cups beef stock
1 bottle Guinness
1 bay leaf
3 tbsp chopped fresh thyme
1 tsp brown sugar
2 tbsp grainy mustard
1 tbsp tomato paste
s & p to taste

Sear meat on all sides. Remove and place in slow cooker. Add to pot - celery, potato and carrots to pan, cook 2-3 mins. Add onion, and remaining ingreds except flour, cook another min. Add this to slow cooker. Add to pot - flour, stock and Guinness – bring to boil. Pour over all into slow cooker. Cook for 3 hours on medium.