Say "crust" and you immediately think "fat", not so - this uses flaky phyllo and spraying with nonstick cooking spray and coated with Dijon mustard this recipe keeps the fat to a minimum.
In a large nonstick skillet, combine the onions, scallions, and broth. Bring to a boil, reduce to a simmer, cover and cook until the onions are tender, about 10 minutes. Uncover and cook until the liquid has evaporated and the onions are golden, about 5 minutes longer. Remove from the heat.
Meanwhile, place the flour in a medium bowl, and gradually whisk in the milk until no lumps remain. Whisk in the whole eggs, egg whites, and cayenne pepper until will combined. Set aside.
Place 2 phyllo sheets in a 9-inch tart pan with a removable bottom, overlapping the sheets at right angles and tucking in the edges. Lightly spray with nonstick cooking spray and brush with 2 teaspoons of the mustard. Top with 3 phyllo sheets, overlapping the sheets at right angles and tucking in the edges. Lightly spray with nonstick cooking spray and brush with 2 teaspoons mustard. Repeat with the remaining phyllo sheets and 2 teaspoons mustard.
Sprinkle the onion mixture and canadian bacon on top of the phyllo, pour the milk mixture over the filling, and place the tart on a baking sheet. Sprinkle with cheese on top and bake for 25 minutes, or until the phyllo is lightly golden and the filling is set.