Chef Matt Millea devised this outstanding dessert for one of our Chef Walks, where it garnered rave reviews from every attendee. If fresh tarragon isn't available, substitute fresh basil or mint. If you have the time and inclination, you can serve the strudel slices with a drizzle of fresh strawberry sauce and a dollop of crème fraîche or whipped cream.
Make the streusel: Place the sugars, cinnamon, and flour in a small bowl and whisk to blend. Add the vanilla and the soft butter, and use your fingers to combine the ingredients, blending until the mixture resembles cornmeal. Set aside.
Make the strudel filling: Place the strawberries, rhubarb, almonds, cornstarch, and tarragon in a large bowl and stir to blend. Add 3/4 cup of the streusel mixture to the bowl and combine. Set aside.
Position a rack in the lower third of the oven, and preheat to 350 degrees F. Line a rimmed baking sheet with parchment paper.
Lay one sheet of phyllo dough on a clean counter or tray and brush with some of the melted butter. Sprinkle the dough with a tablespoon or two of the remaining streusel, and top with another sheet of phyllo. Brush with butter and sprinkle with streusel. Top with the remaining sheet of dough.
Spread the strawberry-rhubarb mixture over the dough, leaving a one-inch border uncovered on all sides. Starting at the short end, gently roll the phyllo to form a tight cylinder.
Transfer the strudel to the prepared pan. Brush the exterior of the strudel with melted butter and dust with the remaining streusel.
Bake until the strudel is golden brown, 20 to 25 minutes. Remove the pan from the oven and cool on a wire rack for 10 to 15 minutes. Cut the strudel with a sharp knife and serve warm.