Place all the ingredients in a saucepan and simmer over a medium heat for about 20–25 minutes. As it simmers, some of the liquid will evaporate and the mixture will thicken. Once the liquid has reduced and the desired consistency has been reached, remove the saucepan from the heat.
Immediately transfer the hot mixture to sterilised preserving jars, ensuring they are filled to the brim to minimise air pockets. Seal the jars tightly and leave them to cool at room temperature. Once the jars have cooled completely, store them in the fridge. When sealed, the contents will keep for up to 6 months.
Place the eggs in a saucepan of boiling water and boil them for exactly 5 minutes. Then cool them quickly under cold running water.
Peel the eggs and place them in a blender along with the cheese, lemon juice, mustard, salt and pepper, and blend until smooth.
Whilst the blender is running, gradually add the oil in a thin stream until it has emulsified into mayonnaise. It is important to add the oil slowly to ensure a smooth and even consistency. Season to taste with more lemon juice and salt if necessary.
If desired, transfer the mayonnaise to a piping bag. Refrigerate the mayonnaise immediately, where it will keep for 6–7 days.
Cut the squash into 1cm-thick slices – two slices for each smørrebrød. Sprinkle with salt and pepper on both sides and leave to rest for 15 minutes. Have three deep plates ready: one with flour, one with beaten egg, and one with breadcrumbs mixed thoroughly with Parmesan and rosemary.
Slice the tomatoes into thick slices. Spread a little mayonnaise on the rye bread, place the tomato slices on top, and dot small amounts of mayonnaise between them. Arrange the red onion rings and crudités on top of the smørrebrød and grate the goat’s cheese over it using a grater.
