Preheat oven to 350°F. In large pot of cold water set over medium-high heat, bring potatoes to boil; boil for 8 to 10 minutes or until almost tender but not fully cooked. Drain; set aside.
In small saucepan set over medium heat, melt butter; stir in flour and cook for 1 minute. Gradually whisk in milk until boiling; simmer for about 5 minutes or until thickened. Stir in lemon juice, lemon zest, mustard powder, salt and pepper.
Layer half of the potatoes in greased 8-inch (2 L) square baking dish. Top evenly with surimi; pour half of the sauce over top. Repeat layers with remaining potatoes and sauce.
Topping: Stir together panko, melted butter and dill; sprinkle over gratin.
Cover gratin with foil; bake for 25 minutes. Remove foil; bake for about 15 minutes or until potatoes are tender and topping is golden brown. Let stand for 5 minutes.
Tip: Use milk with at least 2% fat in this recipe; non-fat milk doesn’t add enough richness or body to the béchamel sauce.