Bring butter to room temperature. Slice avocado diagonally into 1cm pieces and drizzle with lemon juice. Thinly slice yellow bell pepper and onion, soak onion in water, then drain.

Mince basil and mix it in a bowl with mayonnaise, salt, and pepper.

Lay out cooking paper. Spread butter on both slices of bread, then spread grain mustard on one slice.

Layer half of the green leaf lettuce, yellow bell pepper, onion, basil mayonnaise, tomato, and avocado on the mustard-spread bread.