Potato, Cauliflower and Broccoli Casserole with Welsh Rarebit Top

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4 starchy potatoes (about 2 pounds), peeled and cubed

2 large parsnips (about 1 pound), peeled and cubed 

Salt 

4 ounces softened cream cheese 

1/2 to 2/3 cup milk 

1/4 cup finely chopped chives

1 tablespoon horseradish 

Freshly ground black pepper 

1 large egg, beaten 

1 head cauliflower 

1 bundle broccolini, sliced into long florets

4 slices good-quality pumpernickel or white bread 

1 stick butter, plus more to grease casserole dish

2 large cloves garlic, finely chopped 

1 tablespoon hot sauce, such as Frank's RedHot

1 1/2 teaspoons dry mustard, such as Colman's

3 tablespoons flour 

1 cup porter beer, at room temperature

1 tablespoon Worcestershire sauce 

1 teaspoon lemon juice 

1 cup beef stock 

2 1/2 cups shredded extra-sharp yellow Cheddar

Chopped tomato, for garnish

Watercress, for garnish