Lobster mac cheese

Name: Lobster mac & cheese
Ingredients: 1 frozen lobster (around 500g), defrosted; 1 tbsp olive oil; 1 onion, roughly chopped; big pinch cayenne pepper; 1 tbsp tomato purée; 1l milk; 400g macaroni; 50g butter; 150ml white wine; 50g plain flour; 150g tub brown crab meat (or fifty-fifty white and brown); 1 tsp Dijon mustard; 160g extra mature cheddar, grated, plus extra to top; 2 garlic cloves; 2 tbsp olive oil; 50g breadcrumbs; 1 lemon, zested