1
: Cook potatoes in boiling salted water until just tender, about 5 minutes. Drain, rinse under cool water and set aside to dry.
: Filling:
2
: Heat oil in large skillet over medium heat. Add onion, stirring occasionally until caramelized and brown, about 30 minutes. Cool. Toss with remaining filling ingredients in separate bowl.
: Sauce:
3
: In bowl, combine evaporated milk and water. Melt butter in saucepan over medium heat. Whisk in flour and cook for 20 seconds; slowly add evaporated milk/water mixture (about 1/4 cup/50 mL at a time), whisking continuously to remove any lumps before adding more liquid. Stir in mustard, increase heat to high and bring to a boil, whisking constantly. Once the mixture boils, reduce heat to low and cook 1 more minute or until thickened. Season to taste and remove from heat.
4
: Preheat oven to 350°F (180°C). Lightly oil a 9” square (2 L) dish with sides at least 2” (5 cm) high.
5
: Place overlapping potato slices in dish followed by 1/2 cup/125 mL of the filling and 1/3 cup/75 mL of the sauce. Repeat 5 times, ending with a sauce layer on top. Bake in oven for 30 minutes or until mixture is hot and bubbly and potatoes in centre of pan are tender. Let stand 10 minutes before serving.