Preheat oven to 375 degrees F.
In a large skillet, sauté onions and leeks in 2 tablespoons of butter over medium-low heat, stirring occasionally until a caramel brown, about 20 minutes.
Meanwhile in another skillet, sauté potatoes in 2 tablespoons butter until tender crisp; about 10 minutes, stir occasionally. Remove from heat; stir in thyme, tarragon, sour cream, and lemon juice, Cambozola and half the salt and pepper. Add the caramelized onion mixture; mix well.
On a jellyroll pan, fold one phyllo sheet in half lengthwise; brush with about 1 tablespoon melted butter; repeat layering phyllo with remaining sheets.
Brush mustard over phyllo. Spread Cambozola and vegetable mixture to cover half of the length of the phyllo and leaving a 1-1/2 inches border on the short sides. Roll jellyroll fashion; place seam-side down on pan and brush with remaining melted butter. Bake on upper middle shelf 25-30 minutes or until golden brown.
Place roasted peppers in a food processor with vinegar, oil, juice Tabasco and remaining salt and pepper; blend until smooth. Slice strudel and serve on a plate drizzled with roasted pepper coulis.