Open Steak Sandwich with Onions & Barbecue Sauce

Ingredients

• 1 kg piece hanger steak, at room temperature
• For drizzling: olive oil
• 2 ciabatta rolls, halved horizontally
• 2 Spanish onions, thickly sliced
• 2 vine-ripened tomatoes, thickly sliced
• To serve: good-quality mayonnaise
• 1 cup (loosely packed) wild rocket
• Barbecue sauce
• 40 gm butter, coarsely chopped
• 2 onions, finely chopped
• 2 garlic cloves, finely chopped
• 125 ml Kentucky bourbon
• 350 ml tomato sauce
• 250 ml each tomato passata and cider vinegar (1 cup)
• 150 gm brown sugar
• 80 ml Worcestershire sauce (1/3 cup)
• 60 gm molasses (¼ cup)
• 3 canned chipotle chillies in adobo (see note)
• 1 tbsp chipotle Tabasco (see note)
• Juice of 1 lemon