Velvet onion dip - Peter's Yard

Ingredients

• 70g unsalted butter
• 3 medium white-fleshed onions, roughly chopped
• 2 large cloves of garlic, finely chopped
• 1/2 tsp dried rosemary
• 1 tbsp white wine vinegar
• 3/4 tbsp caster sugar
• 1/2 tsp salt
• 4 tbsp double cream
• 2 tbsp milk
• 1 large long shallot, sliced
• 1 tsp rice flour or plain flour