- Flavour the sauce: Preheat the oven to 180°C (160°C fan oven), gas 4. Pour 200ml milk into a pan. Remove the outer layer of skin from the onion slice (but leave on the inner layers as they will colour the sauce). Pare two thin strips of zest from the lemon. Add the onion, lemon zest and bay leaf to the milk and heat to boiling point. Cover the pan with a lid and leave to infuse for a few minutes.
- Prepare the avocados: While the milk is infusing, halve and peel the avocados, and remove the stones. Thickly slice the flesh and put into a bowl. Sprinkle with 2 tsp lemon juice and some ground black pepper, and toss to coat. Arrange half the slices in a baking dish, then scatter with the chopped tomatoes. Top with the remaining avocado slices.
- Finish the sauce: Discard the onion slice from the infused milk (but leave in the lemon zest and bay leaf). Blend the flour with the remaining milk and the mustard, then whisk into the warm, infused milk. Bring to the boil, stirring until thickened and smooth. Discard the bay leaf and lemon zest, then season the sauce with a little salt.
- Bake the dish: Pour the sauce over the avocados, then sprinkle the top with the flaked almonds, cheese, if using, and paprika. Bake for 15 minutes or until the sauce is bubbling and the almonds are lightly browned. Warm some crusty bread for 5 minutes in the oven at the same time to serve with the dish.
*For flaked almonds, mix 25g roughly chopped unsalted peanuts with the Cheddar cheese. *For sun-dried tomatoes, use a 450g jar roasted red peppers, drained. *For flaked almonds and Cheddar, use 25g grated Parmesan or Italian-style hard cheese and 25g crunchy seed mix.
Don't be tempted to use under-ripe avocados, as they will have a slightly bitter taste. Perfectly ripe fruit will be just soft when pressed and have creamy flesh.