Pot-roast beef with French onion gravy

INGREDIENTS

• 1kg silverside or topside of beef , with no added fat (see tip, below)
• 2 tbsp olive oil
• 8 young carrots , tops trimmed (but leave a little, if you like)
• 1 celery stick, finely chopped
• 200ml white wine
• 600ml rich beef stock
• 2 bay leaves
• 500g onions
• a few thyme sprigs
• 1 tsp butter
• 1 tsp light brown or light muscovado sugar
• 2 tsp plain flour