All I can say is … some flavours were made for each other: potato, leek and blue cheese are like a true blue trio! Use the melted butter as a substitute for the olive oil to brush the phyllo sheets if desired. It’s a toss up, butter will colour the strudel better but olive oil will add good flavour.
Tomato Fennel Sauce
Preheat oven to 375 degrees F. Line a baking sheet with parchment paper.
Add potatoes to a medium pot. Cover with cold water. Bring to a boil over high heat. Add salt and reduce heat to medium. Cook uncovered until potatoes are fork tender, about 8 to 10 minutes. Drain and set aside.
Meanwhile, sauté onion, leeks and thyme in butter on medium high until golden and caramelized, stirring frequently. Process will take about 20 minutes. Remove from heat and add the chopped chives, tarragon, sour cream and lemon juice. Combine the leek mixture with the potatoes while both are still warm. Break potatoes up gently with a wooden spoon. Cool mixture and season with salt and pepper. Add the crumbled blue cheese and blend leaving chunks of cheese.
Brush 1 sheet of phyllo with olive oil. Turn phyllo with long side horizontally in front of you. Repeat with remaining phyllo sheets brushing with oil each time. Lay potato and leek mixture along bottom edge of phyllo horizontally, leaving about 1 ½-inches on either side to tuck strudel in. Brush strudel with egg white along all edges to create a seal when baking. Begin rolling strudel like away from you, like for rolling a jelly roll. After first turn, tuck edges in and continue to roll. Brush strudel with olive oil or some melted butter. (Melted butter will give a better colour.) Transfer to prepared baking sheet.
Bake for 20 to 25 minutes or until golden and flaky. Let stand for 5 minutes and then slice while still warm.
Prepare sauce while strudel is baking. Sweat fennel in butter for 3 to 5 minutes or until soft. Add the tomato sauce. Simmer sauce covered until fennel is very tender, about 10 minutes. Season with salt and pepper. Finish sauce with cream, if desired. Serve warm with strudel.