2 large russet (baking) potatoes (preferably organic; 3/4 pound each, scrubbed
3 tablespoons mayonnaise
1 tablespoon plus 1/2 teaspoon water, divided
3/4 teaspoon tomato paste
1/2 teaspoon anchovy paste
1 1/2 tablespoons Dijon mustard, divided
1/2 medium onion, finely chopped
1 tablespoon olive oil plus additional for coating potatoes
1/2 slice firm white sandwich bread
1 tablespoon milk
1/2 pound bulk breakfast sausage (1 cup) or 1/2 pound of breakfast sausage, casings removed
1 tablespoon chopped parsley
Prick potatoes with a fork. Microwave, covered with a paper towel, on high power until tender, 10 to 12 minutes.
Cutting lengthwise, remove top fourth of each potato and scoop out centers (reserve for another use), leaving 1/2-inch-thick walls.
Whisk together mayonnaise, 1 tablespoon water, tomato paste, anchovy paste, and 1 1/2 tablespoons mustard.
Preheat oven to 350°F with rack in middle.
Cook onion in oil with a pinch of salt in a small heavy skillet over medium heat, stirring occasionally, until golden, about 10 minutes.
Meanwhile, tear bread into small pieces, then soak in milk in a medium bowl until very soft. Mix in sausage, parsley, onion, and 2 tablespoons mayonnaise sauce with your hands.
Divide filling between potatoes, then coat skins lightly with oil. Bake in a 4-sided sheet pan until sausage is cooked through, about 30 minutes.
Remove potatoes from oven and turn on broiler. Spoon remaining mayonnaise sauce over filling and broil 3 to 4 inches from heat until charred in spots, 3 to 5 minutes. Cool slightly.