Preheat the grill to high.
Simmer the shallots and port together in a pan until almost all of the port is absorbed into the shallots. Allow to cool.
Melt the butter in a medium pan and stir the flour through to make a roux (paste). Remove from the heat and gradually add the milk, drop by drop, stirring with each addition to avoid lumps. Return to the heat and simmer for 2-3 minutes, stirring constantly, until the flour is cooked out and thickened the sauce.
Add the stilton and cheddar cheeses to the sauce, stirring to melt. Remove from the heat and stir the mustard, egg yolks and cooled port shallots through to combine. Check for seasoning.
Spread the rarebit mixture generously onto each of the six slices of bread. Place on a baking sheet and grill for 1-2 minutes until golden and bubbling.
Serve the rarebits immediately with some dressed watercress.